Swamp hills doesn't naturally generate seagrass

The bug

Swamp hills will generate normally with fossils, blue orchids, and swamp villagers (with exceptions like MC-30587 and MC-85975), but they will never generate seagrass.

How to reproduce

Use the seed -627636422679431 and look at the swamp hills near spawn, or teleport to 1000 77 300 to see that swamp hills never generates seagrass. Seagrass will still grow in swamp hills when bonemealing, however.