Uninvited instruments popping up in the mixer.
I have experiences several times that additional instuments are popping up the mixer. For instance I have a Acoustic bass at the bottom bar. Then suddenly there are 4 extra bass instruments in the Mixer (Acoustic bass-arco, Acoustic bass-tremolo, Acoustic bass-Slap + Acoustic bass-pop).
These extra instruments are not shown in the score and not in the list when I go to Edit Instruments. Hence, I do not manage to delete them.
I don't understand where they come from or how I can get rid of them :-(
Comments
They are mixer channels assigned to that instrument and are always created when that instrument is added. You can change the way the bass is played using staff text, and these mixer channels are what make this possible. See https://musescore.org/en/handbook/mixer#mid-staff-change for more info on doing this.
FYI, in version 3, these extra channels will be hidden unless you click an expand button.
It might help to keep in mind the Mixer isn't showing instruments, it is showing channels. Stringed instrument like the bass include three separate channels, so that you can switch between arco, pizzicato, and tremolo at any point within the score. For 3.0, there are also slap & pop, so I guess you must be using a 3.0 build of some sort, but not a recent one or you'd be seeing them collapsed into one column. My guess is you are using one of the alpha builds, from before the channel collapsing was implemented.
For now, just ignore the channels you aren't using - there is nothing to be gained from attempting to delete them. And be content they are there should you decide you want the bassist to pick up a bow at some point.