Comment 8 for bug 1808697

What exactly did you done step by step and what does Mixxx wrong?
How do you manage to set A to > 100 % rate.

I see how this case is odd, however I don't see anything wrong from your screenshot.

The sync feature calculated the half/double factor by using the file BPM. This is done for a good sound at any slider position.

Your case is this:
             A B
Current BMP 210,77 421,54
Rate 100,73 142,27
File BMP 105 174

Since 174 is closer to 210 than to 105, (36 vs 69) the rate at unity A is adjusted to 20 %.
If you now adjust A to 100 rate B becomes 140, which is correct.

Syncing to B to 210 might be expected in this case, but that creates other odd cases in less exceptional cases.