This is a BTRoblox browser extension issue. Roblox’s default 2D/3D “Try On” feature works fine.
This is not related to the asset privacy beta. I’m pretty sure this is because the extension is currently unable to render meshes uploaded in Roblox’s latest mesh format. This happens every time Roblox enables the uploading of meshes in a new mesh format version. Keep in mind that the Roblox mesh format is purely an internal technical thing, not really something that is accessible to devs.
It’s pretty crazy just how many times people report bugs on the DevForum that are actually the fault of a third-party browser extension. So many people heavily rely on extensions that modify the website to the point where they forget what the unmodified website looks like. Roblox should really start taking notes from some popular extensions.