In the case a Roblox user has an item equipped on their avatar that has been moderated. Their avatar will not render on the site. If you were to look on your friends page, and one of your friends has a moderated item equipped. The headshot of their avatar will not render, as shown in the photo below.
The user pictured above has an item equipped of which has been moderated. Thus allowing their avatar to appear ‘invisible’ on the website. Plus when you try to view their avatar on their profile whether it be 2D or 3D. You will be met with with an infinite loading glitch.
You can see that the user has an item equipped that has been moderated. Which allows their avatar to appear ‘invisible’ on the site.
Reproduction
In order to reproduce this bug, you’d need to have an item in your inventory that has been moderated and is content deleted. You’d then need to equip said item and your character should no longer render on the site.
Alternatively: You could also find a user who has a moderated item equipped rather then equipping said items on your own avatars.