Too many instruments in MIDI file created with Musescore (visible after uploading for example to Synthesia)

Hello,
I have sheet music in Musecore just for flute (melody) and piano (bass + chords). I export it as midi.
This midi is loaded into Synthesia. I always make the setting in "Hands, Colors, and Instruments" in Synthesia at the beginning.

I don't know why then there are often a bit too much of instruments. This time it's really much too much - ie there are many more instruments than just one flute and one piano.

I'm attaching screenshot of the beginning of the notes in Musescore and a screenshot of Synthesia.
I don't know why there are so many instruments (I suppose it's a bug) and I don't have the enough strength to figure out so many positions, which of them should I change as flute or as piano.

In the screenshot from Synthesia you can see a lot of different channel numbers - where did they come from? I've never set anything up like this.
Can I do something somewhere earlier in Musescore so that there are as many channels as I have staff or a many as I had instruments (for example flute and piano)?

What can I do to avoid this now and in the next songs?

I will be very grateful for your help
Regards