linked Document not visible although there are linked Assets

Yesterday, I linked the Color Assets from two different XD-Files (all files are stored in the cloud). Today, I realised, that the linked colors are still in the panel, but the document is no longer visible in the filter dropdown (see attachment). If I want to relink the document, I get a message "No assets found in ...", although there are still assets in this document. Also I can't delete the linked Assets, because the only way is through the filter dropdown (like described here). Now I can neither delete the linked assets nor re-link the assets. So I ended up in a dead end.

The reason why I wanted to do this is as follows: I have a lot of different colors and this is very confusing. So I searched for an opportunity to do that, but there is no option (see here). So i came up with the idea to outsource the colors into different files, because then I can use the filter dropdown. In addition, I had the problem that I couldn't define duplicate Colors (like described in here). With this solution, this would no longer be a problem.  So I was able to kill two birds with one stone (awesome!).

I'm working with Adobe XD Version 29.3.32.3, Creative Cloud Sync 4.3.45.2 on a windows 10 Enterprise Version 10.0.18363.