While trying to work out why OtakuNekoToT's own Propeller bug was happening, I discovered a few other bugs that may (or may not) be related...

#1 Modeled Airfoil Faces the Wrong Direction
This only happens when Reverse Rotation = False (Default)
If viewing the propeller from the FRONT, when set to False, the blades and hub rotate Counter-Clockwise. However, the flat side of the blades is facing forward, not backwards, meaning the airflow would push the plane in reverse..... that is, if the model dictated the way the physics in game work.

Thankfully, the model does not dictate how the engine or propeller work, and this backward-blade has no functional impact. At least on the game's surface? As I wonder if it doesn't by chance have something to do with why OtakuNekoToT is experiencing what they were.

The visual solution for it is to set the engine model's scale to -1,1,1.
I tried setting just the Hub, but changing any of its scale values -- despite the 'hub' on the Turbo Prop manipulating the ENTIRE engine as well -- always resulted in the curved tips being changed as well (visually, would change its rotation). The only way to keep the proper visual rotation was through the part's main scale as provided.
[Caveat: This one requires it to be done on the backend, because doing that to the Turbo Prop part actually causes catastrophic problems which results in the creation of antigravity, and other changes to quantum physics that the Unity engine isn't able to compute.]

#2 The "2D Sprite" for Blurred Motion of the Blades is also Backwards.
THIS time, the bug occurs regardless, happening whether it's Reverse Rotation = False or Reverse Rotation = True
In this configuration the hub, when viewed from the front, spins Clockwise. The curved tips of the Scimitar blades are therefore at the left of the direction of rotation. However, the Sprite has them depicted as spinning INTO the curved tips, not away-from. (It's hard to see, it's so subtle an effect, but becomes more apparent in slow-mo, or readily obvious when simply pausing the game.)

In other words, the Sprite just needs to be rotated 180 around the X (or Y) so that it's spinning the same direction but is depicted opposite as it is currently.

In terms of severity, both are low, obviously.
Though, #1 is relevant to adding VR support, given people will become far more intimately close to the builds through a VR environment. As such the reversed Scimitar airfoil would be easier to spot. The blurred prop sprite had gone unnoticed by me up until now, because only now was I really studying it in an effort to figure out the bug in the other thread.

So even with VR, it's unlikely to get noticed. I only included it here just in case it is relevant to what might be causing the other issue(s).