Light blocks can stop fluids as if they are an actual block, like a barrier or structure void.
This may be intentional since you cannot waterlog flowing water blocks
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I can confirm. On bedrock the light block gets waterlogged by flowing water. On java no blocks can be waterlogged by flowing water, so it may be intended.
Intended / feature request. This is how waterlogging mechanics have always worked.

Noticed also not able to get farmland if there is a lightblock "above/ontop" of it