Also not all service providers support RCS anyway, so that may be completely irrelevant (if you want to test this, Google's Messages app supports this: it calls it 'chat', and the app's Advanced settings contain options for this. But of course you'd only have been able to see them in the first place if the app you were using at the time was one that supported RCS, so unless you'd then changed to one that doesn't that wouldn't explain it (and I'm pretty certain that Textra does not support this, so switching from Textra to the stock app wouldn't fit this). There are some types of messages which not all apps can read (RCS, also known as Advanced Messaging), so if by chance that had been used it would be possible for some apps to see the message and others not. You could just try installing another SMS app and see whether it can see them, just to be sure. If they were stored within an individual message app the other apps would not be able to see them.

It stores them in the database of a 'call and message storage' app, which is why when you install a new message app it can straight away see all of your messages. It doesn't store messages in the SMS app at all. In that case it's just a question of whether you have any backups, since there's no way of recovering them otherwise. Of course if your pre-installed message app also can't see them then it's a different problem, as that would imply that they really are gone.

And that is what I did to fix it, though I expect that wiping data for Textra would have had the same effect. I don't know why, but I noticed the other day that a recent message wasn't visible in Textra, even though the older messages in the thread were (and all were visible in the other message apps I have installed - yes, I don't know why I have three, magpie tendencies). Feel free to back up the messages first if you want, but they aren't stored in the Textra app itself, or any SMS app, but in a separate message storage app, so as long as you don't clear data for that app you should be fine. In which case uninstalling and reinstalling Textra should force it to resync and solve the problem (though you'll have to redo your personal customisations and settings). Can you see them using the built-in app? If so then they are still in the database, and it's just Textra has got out of sync.

App textra