The bug
Block hitboxes are no longer visible behind transparent blocks, such as stained glass or water. This doesn't apply to normal glass blocks.
Related issues
discovered while testing
is duplicated by
testing discovered
Attachments
Comments

[media]

I'm unsure if the following was ever intended behavior but this bug also prevents the entire hitbox of a transparent block from being seen.
Ex: If you look at a block of ice in a lake, you see a square above the ice, but not the other sides of the hitbox, which used to be visible.

Actually back as of 1.16.2. MC-198103 However sadly that is resolved as won’t fix.