Instead of reflecting the sky/fog, handheld blocks use the brightness of wherever the player is standing. This works in most cases, but when reflecting, for example, distant objects while being under a roof / under trees, it looks like if the sky was really dark, while the rest was sunlit.
This can't be fixed for technical reasons unfortunately. It's there to prevent issues like held items reflecting brightly inside closed areas like caves
Instead of reflecting the sky/fog, handheld blocks use the brightness of wherever the player is standing. This works in most cases, but when reflecting, for example, distant objects while being under a roof / under trees, it looks like if the sky was really dark, while the rest was sunlit.
Video: https://imgur.com/a/tFEW5ZG