Are you sure a similar issue hasn't been opened yet?
Yes.

Describe the bug
Attempting to saveinstance any game with something like a MeshPart or UnionOperation where the object contains the CollisionFidelity property, the saved game will always have any object's CollisionFidelity set to Box, regardless of whether or not that's what it's truly set to.

To Reproduce
Steps to reproduce the behavior:

1. Go to any game with a MeshPart or UnionOperation.
2. Save the game and open it in Roblox Studio.
3. Select any object with a CollisionFidelity property, and look at the value it's set to in Studio.
4. You will find the object's CollisionFidelity set to Box, regardless of what the true value is in game.

Expected behavior
The object's CollisionFidelity should be set to its correct value instead of always being Box

Screenshots
A UnionOperation's actual CollisionFidelity, screenshot from inside Dex:

That same UnionOperation in Roblox Studio:

Executor (& Emulator if used) Name(s):
Swift v1.5.7b

Game Link:
Any

Script Used (Options matter):

local Params = {
 RepoURL = "https://raw.githubusercontent.com/luau/SynSaveInstance/main/",
 SSI = "saveinstance",
}
local synsaveinstance = loadstring(game:HttpGet(Params.RepoURL .. Params.SSI .. ".luau", true), Params.SSI)()
local Options = {}
synsaveinstance(Options)