I've never seen this happen, because (as far as I can tell) all KeePassXC clients auto-save the file any time a change is made, and all the Nextcloud clients auto-sync as soon as the file changes. Keepass is also resilient to the underlying file changing while you have, say, the edit password dialog open.

If a conflict did happen though, newer versions of Nextcloud just keep both copies and alert you to resolve it. If I had to resolve this I'd probably try the built-in database merger first: https://keepassxc.org/docs/KeePassXC_UserGuide#_merging_data...