Alright, so here's an update: On a blank new project, everything seems to work as intended! There's no corrupted data happening. I guess it must be a plugin incompatibility in that case. I'm just going to blame the VisuMZ plugins I have installed in my other project, they're usually the culprit. :P
Hmmmm, well, I'd be very curious to know the cause of the issue. The editor run in its own environment and its interactions with the runtime playtest and other plugins are extremely limited. Still, it's good to know that it doesn't happen on a new project.
Thanks for the update, I'll run some more test on heavy-on-plugins projects on my end :)