You should post your phone, the cable and the charger (specs) Also the soc at the beginning of the charging process.

I am using Joplin on Syncthing with 3 devices I have edited one note on both devices before syncing, so the note got duplicated. (Joplin is syncing to a folder, then the folder gets synced). This takes a little longer to get synced if I boot and open Joplin directly (as Joplin’s first sync still syncs the outdated files). But mostly works fine.

I am also using Joplin on 2 devices and sync them over WebDav (on NextCloud). As I am editing 99% on one device only, I cannot tell you much about it.

If you are working offline some time I would try syncthing first. I have set my phone to auto usb tether so it would always sync when connected.