piece imported from MIDI acquires multi-measure tied note as second voice in a single staff

As a help for our ensemble, our director sends out midi files as well as scores before we start rehearsing. Normally, I import each into MuseScore as a convenience for things like counting measures. This time, several pieces have sections where what should be a single melody has been joined by a held note in the same staff, usually (but not always) coming off an existing tie. The MIDI plays correctly, and I can import it into NWC (my other notation editor) and it comes into notation correctly, without the held note. Is there a known bug, and a solution? I need to ask the creator of the score before attaching anything.