Is use tempo change only one time and there is no severals turn jus one turn. I don't know what you mean by "is already set to relative -50%".

When I start the song it using the correct "general tempo" and when it come to the bar with tempo change at -50 it half the tempo as expected.
Now, if I start the song (changing nothing else) from the bar where tempo change there is a bug and the tempo is not the half of the g"eneral tempo". I don't know why and I can't call it "working correctly". It's not sounding like this to me.

By the way it's look strange having the tempo half at each turn. I didn't try this but if I need severals turn I hope having a solution in order to avoid this. It's weird I don't know any song with tempo reduced to half at each chorus.

MacMini M1 16Go (Monterey) - Macbook Air M1 8go (Big Sur)
Band-in-a-Box UltraPAK 2021