This is inconsistent with regular tall grass, which has an item form for the single block and the double block form.
Linked issues
is duplicated by 10
MC-266918
Missing item variant for Tall Seagrass
Resolved
MC-126195
Tall Grass appear in the creative inventory, but Tall Sea Grass doesn't
Resolved
MC-126566
Can't use Pick Block to get Tall Seagrass
Resolved
MC-129707
in the inventory of creative mode there is no block of tall seagrass
Resolved
MC-132757
Tall Grass has an item for itself while Tall Seagrass doesn't
Resolved
5 more links
can confirm in 1.13-pre5