Apologies for the long title but that’s basically what is happening here! The camera will basically render over transparent glass instances which exists inside viewport frames with their adorne set to a transparent glass instance!
For whatever reason this also only happens in roblox studio.
This actually seems like it’d be a pretty cool effect if it were supported in the client. We really need some native solution for rendering the CurrentCamera (or any other camera in Workspace for that matter) in a ViewportFrame. In-case any staff member stumbles across this; please make this a genuine feature through a less buggy solution, if possible. Projecting the CurrentCamera onto a part in a ViewportFrame is incredibly cool!
As for how you achieve it, you seemingly have to have the “Unified Lighting” beta feature enabled. That would also make sense since the feature has unfortunately had some other glass rendering issues afaik; aswell as it also explaining why it doesn’t happen in the live client (that beta feature isn’t supported in the client yet, afaik).
It also appears that the beta feature sometimes shows itself as enabled, despite not actually being enabled; I had to toggle it off and on again in order for it load for me but that could have also just been a transient issue on my side of things. Could explain why you couldn’t find the cause though.
In-case any staff member stumbles across this; please make this a genuine feature through a less buggy solution, if possible. Projecting the CurrentCamera onto a part in a ViewportFrame is incredibly cool!