When looking at a block, the hitbox causes semi-transparent blocks behind it (such as water, honey blocks, or stained glass) to not render.
This could be easily fixed by simply making the hitbox an opaque gray, rather than a translucent gray.
Related issues
is duplicated by
Attachments
Comments

[media][media][media]

Relates to MC-166717
Not fixed in 1.16 Prerelease 1

Can confirm not fixed
EDIT: is fixed if not using fancy or fast graphics

Now tracking this issue for the fancy/fast graphics settings in MC-187598