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.
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MCPE-153951
Seagrass & Tall seagrass blocks are 1 pixel too low when placed
Open
1
(Unassigned)
765630
Confirmed
Multiple
Windows 10, PlayStation 4 latest software update
26.50.24 Preview, 26.50.26 Preview, 26.50.27 Preview, 26.60.21 Preview, 1.21.120.25 Preview, , 26.1, 26.2, 26.40, 26.44 Hotfix, 26.45 Hotfix
Retrieved
I can confirm. It's easy to see on mangrove roots.