Key signature does not show for some instruments on this score

EDIT: I resolved my own problem, see comment below

I downloaded a score from https://musescore.com/leonardoptxr/swan-lake-complete-ballet-2 (also attached here). When opening it with Musescore (version 3.6.2.548021803), I noticed an issue.
The horns in F and the Trumpets in E don't seem to be showing key signatures no matter what I do.

Steps to reproduce:
1. Open the score in musescore
2. Enable "Concert Pitch".
3. Notice that at the beginning of the score, every instrument is now showing a key signature of three sharps, EXCEPT the instruments mentioned above do not show any key signature.
4. Also try changing the key to something else by dragging a new key signature from the pallette onto one of the first few bars. Note the affected instruments still do not show key signatures.

Is this a bug, or the result of setting on the instruments in this particular score that I can change?