I encountered a bug where a file I'd been working on for 2 days suddenly abandoned my tempo setting at the 5th bar, suddenly playing back really slowly.
Of course, I'm on deadline... this is really no good.
I found a solution. Clearing midi data didn't do it.
Changing the setting in Playback Controls>HP Preferences>Midi Data>Tempo>[dropdown] fixed the problem. Changing it, then changing it back, cleared the glitch.
The default is HP (Ignore Data). This is what broke. Changing it to "No HP Effect" fixed it - and the bug cleared. I switched back to "HP-Ignore Data" and it was back to working.
Mac OS High Sierra