Repeated expressions and special characters in Musescore 3

This happens with any large selection adding a dynamic, articulation or ornament. If you click on the longest note in the selection then shift click the shortest note you can apply the item to only the selected notes. So if you select a 1/4, then 4 8th the a 16th note using shift + click you can add the item to the notes. As soon as the length of the note gets longer, then everything on the longer note's beat gets selected. So If your staves have first beats that look like

4
8
8
16
16
32
8
4

You will want to select the top 1/4, then shift click the 32nd note you can apply the dynamic to all of those notes at once. If you click the 8th note below the 32nd, the additional beats will be selected in each measure and the dynamic will be added to those notes or rests also. This makes it quicker to apply the item to several notes at once. Often the notes are in the opposite order of this by section in an orchestra so you would click the bottom one then shift + click the top note you want to apply the dynamic to.

This works on any beat.

I hope this makes sense.

In your sample I would have clicked the C in the bass then shift + click the E in acc 1 to apply the ppp to the notes.