I'm sure there are bugs but there are also four things that drop MC in 10 man prematurely:

1. using one of your regular abilities. As a multiboxer I tend to spam my F (follow main) and 2 (main dps/tank key) a lot without even thinking about it, and they are "off" my main bar which turns into the possession bar during MCs. If I don't turn the bar that has these other keys to become disabled during possession and I hit them during MC, the MC always breaks. Stuff in Bindpad, Macaroon macros, main key bindings, etc that you click for abilities on alt and is also bound on your tanks will break MC.

2. Going out of range. In 25 man priests the further priests are from their pets the easier it is for MC to break every pulse. In 10 man, I think the distance is calculated from the initial possession point, so moving around too much will break MC.

3. spam clicking the pole in 10 man. Since there is server time involved just click once, if you spam click you MC, unMC instantly and it gets messy. After an MC break wait a second for the server to recognize that you are no longer MCing too, I find if I click instantly after an MC break I almost never get it and it gets messy.

4. I do this on purpose, but doing /dance (and probably other emotes) breaks MC. I will /dance every 40-60 seconds when the other tank picks up Raz, then I wait one second and re-MC.