I'm playing in 1.15.2 and I have the same bug, and now the texture of the sun is in a map.
I reload the textures and the sun become to the normality, but not the map.
Then I closed the world and opened it again. Every things are ok except the Sun size. It becomes huge. Reloading textures returns it to normal.
I have the same problem in Bedrock 1.14.60 for Windows 10.