Editing a note on a multi-part (mult-line? multi-voice? I don't know the correct Musescore terminology) instrument

Just a note on how I use Musescore so my question is little bit more clear:

1) I import a pdf from a hymnal to the Musescore website using the import feature
2) Musescore provides a .mscz file that can be opened in the software
3) I then copy the imported music lines into a score that contains instruments "piano" and "trumpet". The treble clef of the piano "instrument" is copied to the trumpet "instrument" which has 2 lines, the main melody line (Trumpet 1) and the alto line (Trumpet 2).
4) I then separate this score into parts so each instrument has its own page. Piano is on one page and Trumpet 1 and 2 on another.

The issue usually happens when I try to edit the Trumpet 1 and 2 lines on the Trumpet part page. Particularly, if I try to edit a Trumpet 2 note, it also changes the Trumpet 1 note in the same measure to whatever I edited on the Trumpet 2 line.

What tends to happen is that if there is any change required to a note, I have to delete the Trumpet part page, make the change on the main score sheet, and then recreate the Trumpet part page. This requires the extra work of renaming each line, making sure the stems are in the correct direction, page breaks, and any other edits I previously made that didn't get recreated.

Am I doing something wrong? I've attached an example of an intended result. Editing the Trumpet part is where I usually run into issues.