Haven't experienced this at chain 21, but did lose the chain at 26 last night. Could have sworn I killed the monster faster than the last, but still lost chain. Idk. lately, getting over 30 has been a lil tough for me on BLU in Woh.
Ya, like the first few times I thought it was me, then I started timing my kills. EVERY time I kill a mob after 21 my chain breaks. This wasn't the case when I was with my buddies in a real party though.
I believe as the chain gets higher you get less time to kill the mob to keep the chain. Also I think the level of the target determines how long you get too.
Max CP Chain is based on the level of the mob; it works differently than a normal XP chain. Your chains topped out in the group not because you were killing more quickly but because the targets were higher level.