Simple question

I just downloaded musescore and watched online tutorials, but I can't find the answer to a question that seems as though it would have a simple solution. I am very confused as to why this is happening.

I need to be able to open .mid files (from other software) in musescore, then print them. When I load a .mid file, things are often a bit messy, but can mostly be cleaned up--except for one thing I can't figure out.

In the picture below, I have a few triplets in a measure, but for some reason, when I opened the file with musescore, musescore decided to separate some of the notes from the others and put them in the treble clef. I know how to manually move notes into the other clef, but when I do this, it doesn't join them with the other notes to make a nice triplet. Why is that? How do I join them and make triplets?

In short, when I open .mid files in musescore, musescore often puts notes in the wrong clef seemingly at random and I was wondering why this is happening and whether it could be fixed.

Again, I'm new to this and this is probably a really basic question but I CANNOT find the answer online. Very frustrating since it's such a simple thing, and there's nothing I can do to fix it. (aside from manually deleting and re-inputting every note)

Thanks!