Transparency mask bug
Pasting objects from the clipboard inside the Transparency mask mode doesn't work when the "Paste remember layer" feature is turned on inside the Layers panel.
Arseny, can you please check if it is still a problem for you in the latest Illustrator? It feels like it’s been fixed and the art gets pasted fine with the 'Paste Remembers Layers' option enabled. Even when I copy objects from different layers — it works OK.