Stray accidentals when creating a part

Every so often, when I create parts, stray sharps or flats or naturals appear in the first measure.

I'm working on a piece just now (Piazzola's Milonga del Angel) that I downloaded from MuseScore in a piano version. I've arranged it for three recorders.

When I created the three parts, a stray flat and natural appeared in the alto part (see attachment). Then when I clicked on the flat, all three staves were rejoined in what was ostensibly the alto part (see second attachment).

I'll have to print out the alto part with the distracting accidentals - or print out all three parts together for the alto player.

The bass and C bass parts were not afflicted in this way.