If you place a seagrass on a block you won't notice the problem however, if you place a seagrass on a barrier block and look at its hitbox (of the seagrass) you will notice the block is one pixel too low.
[media]
MCPE-153951
Seagrass & Tall seagrass blocks are 1 pixel too low when placed
Open
10
(Unassigned)
765630
Confirmed
Multiple
Windows 10, PlayStation 4 latest software update
1.21.120.25 Preview, 1.21.110.20 Preview, 1.21.100.23 Preview, 1.21.100.22 Preview, 1.21.90.28 Preview, ..., 1.21.81 Hotfix, 1.21.93 Hotfix, 1.21.94 Hotfix, 1.21.111, 1.21.112 Hotfix
Retrieved
I can confirm. It's easy to see on mangrove roots.