The current state of affairs on iOS 11 is that the beats come about 2 ms too fast until the 15th beat, which comes 30 ms slower as a correction. So the overall tempo over time should be correct. I believe this is a bug in iOS, which I've filed with Apple, but they haven't offered any solutions yet.
At 120 bpm, a 2 ms variance would change the tempo by less than 1 bpm, not the 4+ bpm changes you're seeing.
I have seen variations of up to 5 bpm when using the MD-BT01, but those are blips, not consistent. Anyway, you might try with a wired MIDI connection to see if that helps.