Starting in 1.21-pre 1, when flowing water is converted into a source block, it can form ice even if there is only water next to the block.  This causes ice to form in the middle of pools that are refilling with water.  

To replicate:

1. In a snowy biome where ice can form, make a rectangular pool of water.  Above all of the water edges, block sky access so that ice cannot form anywhere.
2. Change the tick speed to something very high.  Since the edges of the water are covered no ice should spawn.
3. Use /fill to remove all but two sides of the water.  As the water refills, ice will form in the middle of the pool.

Observed Result:

Ice is able to form in the middle of a pool of water.

Expected Result:

Ice is supposed to form when the water source block is adjacent to a non-water block and has access to the sky.  This bug causes issues when trying to refill pools of water in snowy biomes.

Video from SchimmelGelenk: https://youtu.be/IflL9u0yTrU?si=q_g7DgLXQnlJ67gs