Playback sounds like multiple songs being played at the same time

I have been creating lead sheets with chords over the melody notes for months without issue. Yesterday I took my custom template - nothing fancy - and created two short songs.

One plays perfectly sounding clean chords as they occur in the short 18 measure song. The other sounds like complete chaos! It's 16 bars in length with mostly simple chords. I've closed the file; closed Musescore, and even transposed the piece - but nothing fixes the problem: a simple "C" chord sounds like 3 different chords...with different rhythms, et al.
I just found a fix: 1) insert a new measure either before or after an existing one; & 2) copy the entire bad-sounding measure into the new measure - and Voila! it plays well.
Still, what a nuisance!

In the attached file I've copied measure #1 into a new measure (#2) and the old #2 into a new measure (#3). Everything else is left as-is.
If anyone has a suggestion as to how to either: 1) avoid this going forward; and/or 2) a simpler way to correct it...rather than replacing each measure..., I would love to hear it!
Thanks!