I only mention it because I run on exactly the same hardware as you, and have had issues when trying to run too many things. Also, if connecting via SSH using Visual Studio Code to configure files and rules, quite a bit of RAM is used due to the remote server - which has tripped me up in the past!
Do you have any openhab or system logs from around the time that stuff disappears?
How long have these rules been running fine for? Have you added a new rule recently? Have you started using a new binding recently?