The bug
One would expect to be able to target the block below a cauldron through the block the cauldron is in, since you're aiming at an empty air space. This bug was fixed for hoppers and anvils in 17w47a, but still affects cauldrons. See attached pictures.
Related issues
relates to
Attachments
Comments
migrated
migrated
The difficulty of fixing this bug for cauldrons and beds is that the blank spaces under them vary from different perspectives, so extreme amount of calculation in-game is acquired in order to let the targeting work as you said. On the other hand anvils don't have the same issue, for the spaces around anvils are always the same from whatever way you look (I don't know much about hoppers), so the same kind of bug for it was easily fixed.
migrated
That's false, anvils and hoppers are actually much more complex than beds or cauldrons are, and they are not the same from every angle. If they can make it work for anvils or hoppers, it should be a breeze to make it work for cauldrons or beds.
migrated
I mean, the blank spaces for anvils are around them, but those for cauldrons and beds are largely inside (under) themselves.
CreeperMagnet_
Can confirm.
muzikbike
Relates to MC-137336
muzikbike
Seems to have been fixed in 18w13a