Reasons for threads not being resolvable

We are currently using GitLab Enterprise Edition 13.0.0-pre

When doing code reviews on “WIP” merge requests, we start threads by adding comments to code lines. For some of these threads the author of the comment es well as the developer see a resolve button, but for others now.
Also, in the top right area when viewing the merge request the number of unresolved and resolved threads is stated, and it does not match the overall number of threads that are on the page. E.g. it says “0/1 thread resolved”, but there are three threads on the page. Accordingly, only one of the threads has the “resolve” button, but the other two don’t.

We did a session (developer, reviewer and other user that also has access to the merge request), and it seems that this applies to all of the three user types. If there is a resolve button for one user, then all others can see and use it as well. And if there is none for one user, no user user has it. So it does not seem to be connected to the user role in regards to a merge request.

Would anyone have an idea what the reasons are for this? How could threads that do not show the resolve button be resolved?