The bug
Sea grass and tall sea grass are able to grow in swamps and rivers. Sometimes the water is too shallow, causing the sea grass to be partially above the water surface.
To reproduce
Seed: -5972628229347000504
Swamp
/tp @s -221 62 149River
/tp @s -403 64 443
From the article on the snapshot: "Changed generation rules for kelp and seagrass". Is it possible that this new behavior is intentional, but just not explicitly stated?