I went to Google to see what it had to say about individual disappearing messages.
Gemini said,
If individual messages are disappearing from your SMS threads, it's most likely due to accidental deletion, a glitch in your messaging app, a software update issue, or potentially a feature on your phone that automatically deletes old messages to save storage space; check your settings to see if any such feature is enabled. [1, 2, 3]
Possible reasons for disappearing messages: [1, 2, 4]
- Accidental deletion: You or someone else might have accidentally swiped to delete a message within the thread. [1, 2, 4]
- Messaging app bug: A bug within your current messaging app could be causing messages to disappear intermittently. [1, 2, 5]
- Software update issues: Recent phone or messaging app updates might have introduced bugs affecting message display. [1, 2, 5]
- Storage management feature: Some phones have settings to automatically delete older messages to free up storage space. [2, 3, 4]
- "Delete old messages" setting: Check your messaging app settings to see if there's a feature that automatically deletes messages older than a certain time frame. [2]
We can count out accidental deletion just because the odds of you doing this in multiple apps seems slim. I think a bug could be possible. You say, but I tried different apps from fresh installs, ah yes but the way I've known SMS to work is everything goes to through the preinstalled app. The pre-installed messaging app cannot be disabled or uninstalled and I believe this is because it does all of the heavy lifting in messaging and whatever other app that is installed is just icing. Window dressing to make things look pretty, or provide some enhancement. In this case Samsung Messages would be the pre-installed app, and other apps like, Google Messages, Next SMS, Chomp SMS, are more like interfaces or UIs. to Samsung's Messages.
I would try clearing the cache for your Samsung Messages app. Settings>Apps>Samsung Messages>Force Stop (if possible)>Storage>Clear Cache.
As far as the other items Gemini talks about I might go along with except you've tried multiple apps and the odds that old message delete would be turned on in each one and then would also appear to act sporadically are low. The same with it being a storage issue.