Quick note for future readers: this issue has been resolved
The Highlight Adornee no longer works in Studio, regardless of the context. (Still works in live games though).
I’ve noticed this issue spontaneously occur yesterday (June 14th 2023) when a feature relying on Highlight Adornee stopped working in Studio suddenly.
Here are some images of me using highlight adornee on the SpawnLocation (both in workspace and in playergui). Neither are working correctly. I’m using a fresh baseplate on the latest build.
And for the sake of reference, if I forgo the adornee and just parent it under the SpawnLocation, it works as expected.