Solved!
NSAttributedString documentation says that default value of NSShadowAttributeName is nil, but in this case the button title is drawn with white shadow. Transparent shadow solved problem:
NSShadow *shadow = [[NSShadow alloc] init];
[shadow setShadowColor:[NSColor colorWithDeviceWhite:1.0 alpha:0.0]];
NSDictionary *attrsDictionary = [NSDictionary dictionaryWithObjectsAndKeys:
shadow, NSShadowAttributeName, nil];
NSAttributedString *attrString = [[NSAttributedString alloc] initWithString:_customTitle attributes:attrsDictionary];
[mybutton setAttributedTitle:attrString];