ResetPlayerGuiOnSpawn is being set to true whenever a client joins the game, but the place has it’s property set to false in the properties pane. Trying to teach someone scripting and this bug came up, as far as I know it’s not intended behavior.
Incase it’s a corrupted place, here’s the link: Dagz Testing Place - Roblox
It’s not just TestServer, also online servers right? There’s been a couple of threads about it last year (Nov/Dec) but apparently it has not been adjusted yet. For now you can set the property through a LocalScript on each client when they join as a workaround.
A workaround would be to put a script in StarterPlayerScripts:
game:GetService("StarterGui").ResetPlayerGuiOnSpawn = false
I normally do stuff like this to ensure that the properties are correctly set.