Quirks with starting and stopping scripted animations
Playgroup restarts an animation from the beginning if the same scripted animation was already running. To reproduce, call "Playgroup idle9 1" on a NPC then call it again shortly after. Vanilla behaviour* is to just proceed with the ongoing animation. If you call "Loopgroup idle9 1" twice, expected behaviour is to loop it only once.
A paralysed actor playing a scripted animation doesn't freeze at its current frame when "Playgroup idle" is used ("Playgroup idle" or "Playgroup idle 1" are used to stop a scripted animation and resumes normal AI behaviour). It's an obscure quirk I've never seen used in a mod. Calling the same playgroup again while a scripted animation is frozen like this resumes playing the animation at the frame where it stopped. You can reproduce with this file PlaygroupParalysisTest.ESP with "StartScript PlaygroupParalysisTest" on any unique NPC.
*while these are technically quirks of the vanilla engine, Playgroup was very bugged without MCP and they would have been unusable before its patch.