Digging this issue (or feature?) further placed the positioned asset into a nested sequence, applied the Track Matte Key effect to this nested sequence and then got the desired result.
So let me please edit my initial question: is there the way other than adding unneeded (from the first glance) elements like excess nested sequences into the project? I'm afraid I'll get lost in my project sooner or later this way.
(i would edit my previous post but see no posibility to do that and the "... More" menu is empty)