Bitcoin Node shows sync 100% but it's not

Since the last 6months I have the issue that Bitcoin Node appears not to sync. I restart my Umbrel and then it works again but about a month later I have the same problem again. This has now happened three times.

When I open Bitcoin Node it shows it’s 100% synced but when I check the latest verified block and compare that with mempool I know I’m behind.
I usually discover this when my Zeus app doesn’t connect to my Lightning Node.
I’m running a raspberry pi 4 with 8GB RAM and have a 1TB SSD.
Anyone know what is happening here?

Any reason you think of why your node might drop out? That might cause you’re node to fall behind briefly and regularly
I had this issue once when I had too many apps installed/running, where my node would drop out, and once when my node would overheat before I got a fan for it.