Dropbox is an old trap.
The makers of Endnote specifically warn against having a work database placed in the Dropbox folder bc of sync issues (through pull requests?). So I have mine in ./Documents and at the end of a session I copy it to Dropbox.
Such problems can be avoided by syncing to a webdav server or NextCloud
but the webdav if also not without its delays, Extreme slow sync of "resources"