Change of time-signature leads to change of tempo?

Hello fellow MuseScore users.

I have only recently downloaded MuseScore and I have used it to put down the notes of one of my quartets, which has a time-signature of 6/8 and runs at speed "crochet=90 bpm" (given at the 1st measure of the score). Oddly, during playback I find that at some place where I change to a signature of 3/4, the tempo drops significantly and stays that way even after I go back to 6/8, some measures later. When I consult the "play panel", I see that it keeps showing tempo = 90 bpm and "tempo setting" = 83% (?) throughout the whole score.
Does anyone know what might be the reason for my problem?
And more: How do I set the "tempo setting" to 100%?

(One of my guesses is that, when I started writing the notes as an amature user of the MuseScore software, I may have inputted a tempo change at that measure [with some text editing / tempo], which I later deleted. If so, is there a possibility that the information remains even after its deletion ?)

Thank you in advance.