It seems like NPC animations are being lost in the middle of them happening.
As though the NPC is being reset every half second or so, which disables any animation playing.
Possibly related to an update packet that isn't aware of any currently playing animations?
Can't reproduce... ARM_SWING?
@fullwall ARM_SWING is so fast you can't necessarily see the effect.
Try start_use_offhand_item while its holding a shield in offhand. That lasts varyingly between 0 seconds and half a second.