I've noticed odd MPE behavior with the 1.6.3 and 1.6.6 beta firmwares with Live 11.
I have a Roli Lumi Keys and a Roli Seaboard which are both MPE controllers. When I setup my P6 and Ableton in MPE mode the Prophet doesn't always play notes when I press a key. If I press the same key say 10 times in a row it'll sometimes play and sometimes be silent. It actually does it in a pattern that I figured out. 5 plays, 1 silent, 1 play, 8 silent [repeat].
The way I've got Live 11 setup is:
Roli hooked up via USB and turned on in Ableton Midi settings (and configured as an MPE device)
A empty midi track routed externally to my P6 Desktop via USB:
Midi From: All Channels
Midi To: Prophet 6 [MPE]
In the P6 I've got the Midi Channel config set to MPE.
Whats odd about this is when I open Logic and just play my controller the odd behavior is gone (but its hard to test out if MPE is fully working for me)
Here's a screenshot of my Ableton settings: