The bug
Seagrass can't be planted in a water that is two water sources stack on top of each other.
Attachments
Comments 3
This is a duplicate of MC-130955, an issue marked as WAI.
MC-130273
Sea grass can't be placed in 1 × 2 × 1 of water
Resolved
Fixed
5
This is happening because that bottom water source is actually being deleted by the above water source. MC-128860 is the cause of this (I feel like MC-130530 is a dupe of that one, but it's been marked as "related to" instead, so I'll leave it up to the mods to decide which one this bug report is a dupe of), see my latest comment there (as of writing) for a bit more explanation.
P.S. If you need that 2x1 setup for a map/decoration or something, it'll work correctly if you first place the grass and then put the water on top.