I have been having similar issues lately. I keep losing data from the last notes edited. It seems like it might be related to the Android version. I haven't been able to narrow it down but I've lost data several times lately. I use Dropbox and sync between android, Linux Mint, and Windows. Initially I thought maybe it was just a fluke but it keeps happening. I don't have a lot of time to waste troubleshooting. I have been using Joplin a long time for my journal and this is the first time I have had data loss.
I back mine up routinely by export just in case something goes wrong. Do you have TimeShift or something like that that will allow you to get them back? Also, I have not had anymore issues since I stopped syncing automatically to Android. I think it was timing out in the middle of sync. When I sync it manually and make sure it finishes, it seems to work fine.