The keyboard 'quick search' feature of MC is the primary way I work with navigation and file selection. Unfortunately it doesn't work for files with '.' in their name, as '.' appears to reset the list of typed characters. It also doesn't work for '_' and any other non [a-z0-9] chars, as the quick search option states 'letters'. Personally I consider this a bug, typing '.' should not reset the list of typed characters since it is valid file name char. And the same goes for all other valid file name chars.
See attached screenshots for my settings and illustration of the problem.