If an already existing Highlight inside a model with a humanoid is changed by the client, the changes will replicate to the server.
This only happens in Studio
Script
local Highlight = workspace.Dummy.Highlight -- An NPC with a Disabled Highlight
task.wait(2)
Highlight.Enabled = true
Highlight.FillColor = Color3.new(0,0,255)
-- Visual Changes will Replicate, but Properties are Unaffected
If you check the property value you’ll see that the property was not actually replicated back to the server.
This is a rendering artifact only arising from the fact that the client and server in Play mode in Studio use the same rendering engine underneath, and won’t be an issue in a live game or if you launch separate local test server / clients.