I think it's pretty much the same as how MC-125297 worked. Block updates that happen next to tall seagrass don't allow its water to flow. If flowing water touches it, that specific side will then flow outwards as normal (simply because the flow detects a connected water source), but not the other sides (unless water touches them too). It'll go back to being unable to flow from its respective sides if you get rid of the water connected to them.

I think this might be the only block left that bugs out in this specific way; everything else I tested flowed normally. Didn't test every single variation of waterloggable blocks, but something tells me that tall seagrass is (probably) the only one with this problem. I tested all the coral/kelp/regular seagrass though.