I've just noticed that Google Keep and Missed Notifications Reminder don't make a notification sound any more; they did in the past. I expect this is true of most applications, I haven't checked. The applicaions are set to start automatically, not to save battery, and all notification options are ticked. I saw a post elsewhere saying that with MIUI it had become necessary to enable vibration for sound to work, but it doesn't help me. This isn't the same as the original issue - the Keep notification is displayed for a few seconds, but without sound. I don't know, of course, whether this is to do with the xiaomi.eu ROM.
Re the original issue: nake sure that the application isn't set to save battery, and that it autostarts. Battery saving mode delays notifications
Best wishes, pol098 - Redmi Note 7, 20.12.9 xiaomi weekly ROM