Unfortunately I can’t access the Engine Bugs category, as such I have moved this to Education so people are aware that they should not use accessories in this manner.
Below you’ll find two videos demonstrating the rendering problem, I have already found a solution by not utilizing accessories, but rather just attaching a model.
(Please also take into consideration that these accessories and models have String attributes as well as very high triangle counts, with the morph shown having a tri count over 50 thousand)
Using Models welded to character:
Using accessories welded to character (by calling Humanoid:AddAccessory):
As you can see, in the video utilizing accessories, when scrolling in and out of first person the tween is not smooth, and rather goes immediately transparent and opaque, this is not a bug with Transparency or LocalTransparencyModifier. This ultimately can result in the player’s character not being rendered at all.
Utilizing models welded to the character however behaves exactly as intended.