Yesterday i've found out why some tooltips are not displayed. There are 2 reasons.
1. MC uses 3 different types of tooltips:
a) regular (e.g. as for Button panel);
b) balloon (e.g. as in Button editor);
c) custom (for file/folder information).
2. I've disabled balloon tooltips in Windows configuration
As i can see, the 3d type tooltips don't depend on Windows settings. Maybe it makes sense to use only one type (1st or 3rd) for all elements? For the sake of uniformity and independence from windows settings