I have Google Photos upload and backup both disabled.

But then, I'm pretty sure either Google or Samsung SMS app had a "feature" to automatically delete old messages (for a definition of "old" that was neither specified, nor configurable), and it defaulted to ON on my current phone, likely costing me significant chunk of my message archive (that I dutifully transferred over from the previous phone) before I accidentally found and disabled the switch.

So yeah, could be Google Photos deleting it. Or someone else. I don't trust Android as a platform anymore.

BTW. about this "delete old messages" "feature" - most likely this was implemented for performance reasons. But the thing is, you're unlikely to send or receive enough SMS in your whole life for it to take a noticeable amount of space. The irony here is, I do remember a case where the messaging app would become slow and laggy if you had enough texts stored on the phone - but that was solely because someone implemented the message list as a linked list, thus adding a O(N) multiplier to many GUI operations.