Summary:

If the light block waterlogged by flowing water is replaced by blocks waterlogged only in the source, the flowing water will not be removed, but will simply continue to exist with a "non-waterlogged" block.

With the rest of the blocks waterlogged in the flowing water (lever for example), this works with the /setblock command.

This does not seem to affect the gameplay in any way, but the fact that these blocks cannot be waterlogged simply by placing them in the flowing water indicates that this is not intentional.

Steps to reproduce 1:

1. Arrange the water so that flowing water forms.
2. Place the light block in the flowing water.
3. Replace the light block with a waterlogged block only in a full block of water (pointed dripstone or Amethyst for example).
4. Just place the same block in the flowing water as in the previous step.

Steps to reproduce 2:

1. Arrange the water so that flowing water forms.
2. Place waterlogged in flowing water block (lever for example).
3. Replace using the command /setblock the block with a block that does not waterlogged in flowing water (bell for example).
4. Just place the same block in the flowing water as in the previous step.

Observed results:

Blocks that do not become waterlogged in flowing water may become waterlogged because replacing blocks waterlogged in flowing water does not remove the flowing water.

Expected results:

Flowing water will be removed by replacing the blocks waterlogged in flowing water with blocks that do not waterlogged in flowing water.

Video:

Minecraft 2021-11-18 09-11-00.mp4

Minecraft 2021-11-18 09-08-25.mp4

Image: