diff --git a/spec/unit/pushprocessor.spec.ts b/spec/unit/pushprocessor.spec.ts
index 136ecc56c..122084d92 100644
--- a/spec/unit/pushprocessor.spec.ts
+++ b/spec/unit/pushprocessor.spec.ts
@@ -198,11 +198,12 @@ describe("NotificationService", function () {
     });
 
     it("should add default rules in the correct order", () => {
-        matrixClient.pushRules = PushProcessor.rewriteDefaultRules({
+        const pushRules = PushProcessor.rewriteDefaultRules({
             device: {},
             global: {
                 content: [],
                 override: [
+                    // Include user-defined push rules inbetween .m.rule.master and other default rules to assert they are maintained in-order.
                     {
                         rule_id: ".m.rule.master",
                         default: true,
@@ -298,27 +299,24 @@ describe("NotificationService", function () {
                 ],
             },
         });
-        pushProcessor = new PushProcessor(matrixClient);
 
         // By the time we get here, we expect the PushProcessor to have merged the new .m.rule.is_room_mention rule into the existing list of rules.
         // Check that has happened, and that it is in the right place.
-        const containsDisplayNameRuleIdx = matrixClient.pushRules?.global.override?.findIndex(
+        const containsDisplayNameRuleIdx = pushRules.global.override?.findIndex(
             (rule) => rule.rule_id === RuleId.ContainsDisplayName,
         );
         expect(containsDisplayNameRuleIdx).toBeGreaterThan(-1);
-        const isRoomMentionRuleIdx = matrixClient.pushRules?.global.override?.findIndex(
+        const isRoomMentionRuleIdx = pushRules.global.override?.findIndex(
             (rule) => rule.rule_id === RuleId.IsRoomMention,
         );
         expect(isRoomMentionRuleIdx).toBeGreaterThan(-1);
-        const mReactionRuleIdx = matrixClient.pushRules?.global.override?.findIndex(
-            (rule) => rule.rule_id === ".m.rule.reaction",
-        );
+        const mReactionRuleIdx = pushRules.global.override?.findIndex((rule) => rule.rule_id === ".m.rule.reaction");
         expect(mReactionRuleIdx).toBeGreaterThan(-1);
 
         expect(containsDisplayNameRuleIdx).toBeLessThan(isRoomMentionRuleIdx!);
         expect(isRoomMentionRuleIdx).toBeLessThan(mReactionRuleIdx!);
 
-        expect(matrixClient.pushRules?.global.override?.map((r) => r.rule_id)).toEqual([
+        expect(pushRules.global.override?.map((r) => r.rule_id)).toEqual([
             ".m.rule.master",
             "coffee",
             ".m.rule.contains_display_name",
@@ -327,7 +325,7 @@ describe("NotificationService", function () {
             ".m.rule.reaction",
             ".org.matrix.msc3786.rule.room.server_acl",
         ]);
-        expect(matrixClient.pushRules?.global.underride?.map((r) => r.rule_id)).toEqual([
+        expect(pushRules.global.underride?.map((r) => r.rule_id)).toEqual([
             "user-defined",
             ".org.matrix.msc3914.rule.room.call",
             // Assert that unknown default rules are maintained
diff --git a/src/pushprocessor.ts b/src/pushprocessor.ts
index b13cfc0ca..c88ca40af 100644
--- a/src/pushprocessor.ts
+++ b/src/pushprocessor.ts
@@ -49,10 +49,6 @@ const RULEKINDS_IN_ORDER = [
     PushRuleKind.Underride,
 ];
 
-const UserDefinedRules = Symbol("UserDefinedRules");
-
-type OrderedRules = Array<string | typeof UserDefinedRules>;
-
 // The default override rules to apply to the push rules that arrive from the server.
 // We do this for two reasons:
 //   1. Synapse is unlikely to send us the push rule in an incremental sync - see
@@ -119,6 +115,11 @@ const DEFAULT_OVERRIDE_RULES: Record<string, IPushRule> = {
     },
 };
 
+// A special rule id for `EXPECTED_DEFAULT_OVERRIDE_RULE_IDS` and friends.
+const UserDefinedRules = Symbol("UserDefinedRules");
+
+type OrderedRules = Array<string | typeof UserDefinedRules>;
+
 const EXPECTED_DEFAULT_OVERRIDE_RULE_IDS: OrderedRules = [
     RuleId.Master,
     UserDefinedRules,
@@ -171,7 +172,7 @@ const EXPECTED_DEFAULT_UNDERRIDE_RULE_IDS: OrderedRules = [
  * @param kind - the kind of push rule set being merged.
  * @param incomingRules - the existing set of known push rules for the user.
  * @param defaultRules - a lookup table for the default definitions of push rules.
- * @param defaultRuleIds - the IDs of the expected default push rules, in order.
+ * @param defaultRuleIds - the IDs of the expected push rules, in order.
  *
  * @returns A copy of `incomingRules`, with any missing default rules inserted in the right place.
  */
@@ -182,10 +183,10 @@ function mergeRulesWithDefaults(
     defaultRuleIds: OrderedRules,
 ): IPushRule[] {
     // Find the indices of the edges of the user-defined rules in the incoming rules
-    const incomingRulesEnabled = incomingRules.map((rule) => rule.enabled);
+    const mappedIncomingRules = incomingRules.map((rule) => rule.default);
     const userDefinedRulesRange: [number, number] = [
-        incomingRulesEnabled.indexOf(false),
-        incomingRulesEnabled.lastIndexOf(false),
+        mappedIncomingRules.indexOf(false),
+        mappedIncomingRules.lastIndexOf(false),
     ];
 
     function insertDefaultPushRule(ruleId: OrderedRules[number]): void {
@@ -202,7 +203,7 @@ function mergeRulesWithDefaults(
 
     let nextExpectedRuleIdIndex = 0;
     const newRules: IPushRule[] = [];
-    // Process the default rules by merging them with defaults
+    // Merge our expected rules (including the incoming custom rules) into the incoming default rules.
     for (const rule of [
         ...incomingRules.slice(0, userDefinedRulesRange[0]),
         ...incomingRules.slice(userDefinedRulesRange[1]),