Hello Everyone, after having downed rotface on both difficulties (10/25) i can say that there seems to be a bug with with merging of Oozes.
Currectly, after two small slimes merge they form a big ooze, then when 5 more merge with this big ooze it explodes. A total of 7 Small Slimes
What i've noticed is when you have two Big oozes, made up of atleast 4 slimes each (A Big Ooze with 2 stacks of the Buff), and they get close enough to merge, the combined oozeinstantly explodes, however dealing no damage to the raid.
I hope this helps guilds out there struggling on this rather difficult encounter.