Following issues: 1. Created a Tb collection and added 12 add-ons from the creation page. 2. Open the edit collection page, added ~45 add-ons. 3. Open the add-on list of this collection. Actual result: Add-ons added in 2) missing. Tested again with a single add-on, worked fine.
Recognized that the 12 add-ons collection had been created twice and I added the 45 add-ons (logically) only to one collection. Resolving as invalid.