fix: start timer on message even if message already exists

ts/models/conversation.ts

@@ -901,11 +901,6 @@ export class ConversationModel extends Backbone.Model<ConversationAttributes> {
       expirationMode
     );
 
-    // For some reasons, we want a timer update to "off" to disappear with the previous setting on that conversation...
-    const shouldUsePreviousExpiration =
-      expirationType === 'unknown' &&
-      previousExpirationMode !== 'off' &&
-      previousExpirationMode !== 'legacy';
     const commonAttributes = {
       flags: SignalService.DataMessage.Flags.EXPIRATION_TIMER_UPDATE,
       expirationTimerUpdate: {
@@ -937,8 +932,8 @@ export class ConversationModel extends Backbone.Model<ConversationAttributes> {
       }
     } else {
       message.set({
-        expirationType: shouldUsePreviousExpiration ? previousExpirationMode : expirationType,
+        expirationType: expireTimer === 0 ? oldExpirationType : expirationType,
-        expireTimer: shouldUsePreviousExpiration ? previousExpirationTimer : expireTimer,
+        expireTimer: expireTimer === 0 ? oldExpireTimer : expireTimer,
       });
     }