Unexpected "?" symbols in Windows-1251 encoding

[Letoile]

Hello!
I was wondering if I'm not alone with this small problem.
I have to export data to csv with Windows-1251 encoding, cyrillic.
Firstly, I have tried UTF-8. When open exported files in *nix systems, using, e.g. LibreOffice or text editor, all is going ok, but Windows users got "?" symbols in particular places when they open files in Microsoft Excel and try to encode to Windows-1251.
As the data is used only by Windows users, I changed export's encoding to Windows-1251, so, I always got these unexpected "?" symbols in some places when export.
Sometimes "?" replaces spaces, sometimes it's first symbol of string (but not replaces real first symbol, just stands before).
Any suggestions, please? Is it problem with this library?