Discovered while testing MC-241035, and closely relates to MC-241006.
The bug
The texture used for the sun appears horizontally flipped when seen in-game compared to how it is as a file. This is generally impossible to see without a texture pack given the vanilla texture's symmetry, but becomes all the more apparent when a custom, non-mirror-symmetric texture is used.
How to reproduce
Download and apply the testing texture pack attached to MC-241035
Look at the sun
Expected results
The text inside the image would be readable normally from left-to-right.
Actual results
The text appears flipped horizontally from what it should be.
This also affects the moon. It is an old issue. It is likely that this won't be fixed any time soon because fixing it will break every texture pack that has a sun or moon. Until it is fixed (if ever), I recommend flipping the sun and moon textures in the texture pack to get them to render correctly.