rotation arguments are not tab-completed to ~

The rotation command arguments of /execute rotated and /setworldspawn do not provide any suggestions.

Steps to reproduce:

1. Type /setworldspawn ~ ~ ~ (with a space at the end)
2. Observe how no suggestions are shown
3. Type ~ or a y rotation value
4. It's allowed.
5. Clear chat
6. Type execute rotated (again with a space at the end)
7. Observe how only as is suggested
8. Type ~ ~ or a y rotation and x rotation value.
9. It's allowed.

Expected behavior:
The command would suggest ~ for the current rotation.

Observed behavior:
Nothing is suggested.

Original Description
Coordinate arguments (x, y, z) are tab-completed to ~ if no block is in focus but rotation arguments (y-rot, x-rot) are not tab-completed to ~, or even the rotation of the entity being looked at or the player itself.