How can i stop sharing a file?

[Schmu]

Hi,

This is due to a bug in the app “social sharing via email”.
As soon as you disable it, you have the list of users back you shared the file with and you can unshare again.

Edit:
just found this interesting post.

Can't share files, the section where I should be able to see with whom the file is shared is blank

Fyi, the relevant bug is here https://github.com/nextcloud/server/issues/13656 Also, not only the email sharing plugin, but all “Social sharing via xyz” plugins seem to trigger this, so if you run into this problem, disable all those.