Locked files by one user

Nextcloud version: 13.0.2
PHP version: 7.2

The issue you are facing:

I have a nextcloud-server installation for my family on a distributed server with a group-folder. All can share files in this folder and also delete them. The sharing is working, but everytime one certain user is sharing a file with the rest, the file is downloaded, but can’t be deleted automatically on the server. I’ve found that the file is marked as locked in the database and I do unmark it by running a SQL-command on the database. But that’s only a workaround.
What is strange is that it’s always files from the same user that’s leading to this problem. Files from other users in the same folder can be deleted without any hassle.
Any idea what could be the cause and how to solve this issue permanently?

Is this the first time you’ve seen this error?: N