Hello @germ, thank you for your post and welcome to the forum
And a separate thank you for adding the step by step the process
This is by design. If you re-add the same font, you will notice that it will appear in this collection. Collections remember fonts added to them even if the fonts are removed from FontBase, it case it happens by accident or users want to re-add these fonts at some point in the future. This eliminates the need to categorise them again.
I also pointed out: If a design decision is based on the case of an exception (user mistakenly deletes font from collection), still the number of fonts FontBase shows is wrong after deleting a font from a collection. And this should be corrected.
As a followup question for the majority who do not delete fonts from a collection mistakenly: how does the design of Fontbase accommodate to show the correct number of fonts in a collection? Is there maybe some user-setting, or (context) menu item?