Somehow all shopkeepers (new and already existed) keeps being on fire and then it isn't over and over.

Clip (medal): https://medal.tv/games/minecraft/clips/jPR903RkaogY90hNM?invite=cr-MSxmZlcsMjUyNjc1MjAyLA

I don't know why this is happening it's really weird, but we can't seem to fix it.

It's not anything with skript as we disabled everything and it still did it.

Debug mode doesn't send anything and reloading the shopkeeper also doesn't do anything.

Which server and plugin version are you using? Maybe your plugin version is not compatible with your server version. (this is the usual case in which this behavior can be observed)