In FontBase v2.21.0, when fonts in a collection are deleted (context menu > delete from disk), the font reference is not deleted from the collection. The font will not be visible in the collection anymore, but the font counter of the collection does not change. At least in time, collection information is not accurate.
Hello @PB-CCC, thank you for your post 
Indeed collections remember the fonts they contain if these fonts are removed from FontBase. This is by design so that if the fonts are accidentally deleted or removed and then re-added they will show up again in the collection that contained them without the need to remember which collection contained which font:)
Thank you again,
Yuriy
FontBase Dev Team
Please read my report again and change it back to a bug report.
If a design decision is based on the case of an exception, I may not agree, but that’s fine. But the number of fonts FontBase shows is wrong after deleting a font from a collection. And that’s a bug. So, this is a bug report, not a question.
Understood, changed it back, thank you for the clarification