Edit: just had to reset Studio settings back to default!
I’ve tested this one outside of my game as well, when I change a part’s transparency inside my player’s character the whole model re-loads itself, making it look all messed up for a second or two.
To replicate this, simply run a play test, then change your character’s parts transparency.
This transparency aspect is very important for my project, and I’d like to find a solution for it.
https://streamable.com/8tdubt
Not a replicable bug because changing transparency of parts is a main selling factor of my (working!) gamepass and that doesn’t happen. It might be because you aren’t doing it through a script.
I found out about this with a script 
Hmm, yeah I am unable to replicate this.
Perhaps are you working with Humaniod:Move() by any chance? Or a custom movement script? Perhaps this is whats causing your model to unload:
The link I posted is an empty baseplate.
I found out about this with a custom rig attached to my character, which appears with a tween effect.
It occurs after a certain time lapse (as shown in the video). The transparency changes are correctly executed for like 3 seconds before it happens again.
Hmm, any more details of the custom rig and tween effect along with any other possible causes like plugins?
Good point, when I get back home I will disable all plugins.
The custom rig is simply a character model within your own character model, which is welded between humanoidrootparts (it doesn’t have any humanoid). However this seems to have nothing to do with the issue, since it occurs anywhere.
Alright, updating on the situation.
Turning off all plugins didn’t help.