This is an incredibly dumb thing to be complaining about, given that the missing model isn't even meant to show up in normal gameplay. Nonetheless, texture mapping issues like these are still considered valid, like MC-158295, and - due to the lack of any texture shading in this case - is completely fixable, unlike MC-204171 and MC-105619.

The bug

The missing model's textures are applied in a way in which they do not always line up, causing a more broken appearance than intended. Specifically, all textures are oriented in the exact same way, which works for four of the faces, but the north and south faces in specific ruin the look.

How to reproduce

1. Download and apply the "ErrorCotta" resource pack attached to this ticket
2. Obtain and place a minecraft:terracotta block
   1. Note that at the corners, only two of the three of the texture planes match up at a given time

Expected behaviour

The textures would be aligned such that the corners all line up and the color is the same for all three texture planes at each vertex, creating the illusion that the block is composed of eight single-color sub-cubes.

Actual behaviour

Only two of the three texture planes will match in color at each vertex - the third will always be different.

How to fix

As the missing texture and model are hardcoded (MC-50073), this cannot be fixed directly via a resource pack, however a resource pack can be used to create a flawless version of the "expected" missing model, although it will not carry over to any actually broken models.

The "BetterCotta" resource pack, also attached, changes terracotta to use a custom model, which flips the orientation of the north and south faces such that all eight vertices match up. The result is considerably more visually appealing than only two textures aligning at each corner.

I don't know how the hardcoded model works for the missing model, since it's obviously not in the assets directory, but it shouldn't be impossible to convert the hardcoded model to look like this. Worth noting is that this changes the inventory appearances of missing models, with the black and magenta squares switching places, but this probably isn't an issue.