Thats interesting, and kinda goes against the point of a modlog in the firstplace

Kinda. It is to hold moderators accountable for their actions, but if you're both a moderator and owner of an instance, you are God on you own instance and you can purge whatever the hell you want. In this case, the owners of lemmy.ml are the tankies.

Does an instance’s local copy of the log keep the actions?

A modlog is instance specific and does not get federated, so lemmy.ml's modlog is entirely different from lemmy.world's modlog.

Some mod actions aren't federated to other instances' modlogs. I'm not sure if this is just a federation glitch, or if they're running a modified version of the Lemmy backend that let's them exclude individual mod actions from federation.