Seems like this is related to the bosses being reset. Sometimes when the bosses reset you will see their HP drop down to 1% and then it will go to full health as soon as they are reset. In all 3 clips there is vaan resetting and this happens after, so somehow the same thing is happening to players without the "NPC HP reset" occuring afterwards.
Maybe, but how do you explain Irawk's HP drop then ToffeClubs HP Drop 11 seconds later in the second vid? if it was a case of Van resetting wouldn't it have effected both of them since they were right next to each other, similiar to how Dontyagnomie and Stickydaclowns HP dropped in the first vid when they were close by?