Placing a slab underwater doesn't make it waterlogged automaticaly

probably you are placing it in flowing water instead of water source.

I am pretty sure the whole "box" i'm in were only source blocks since i did /fill

Even if it was a flowing block, shouldn't the water flown inside anyways?

May it be the fact that i used prismarine slabs?

Type of slab does not matter. Water won't "flow" inside a block due to the nature of waterlogged blocks (they can only be waterlogged with source blocks)

Ok 🙂

It looks like this is flowing water, not water source blocks. Slabs can only turn into waterlogged slabs when being placed in water source blocks.

Edit: How did I not notice all the comments before mine, lol