The 1.19.0 updated added shrinking underwater fog over time (i.e. gradually expanding visibility), according to the following changelog statement (cp. MCPE-124282).

• Added adjustable fog effect that can expand or shrink over time. Users will get the sense of eyes adjusting when going into water

However, this was not applied to mangrove swamps. The mangrove_swamp_fog_setting.json file in the 1.19 vanilla resources matches the swampland_fog_setting.json from 1.18.

Steps to reproduce

1. Create a world with the mangrove swamp seed from the seed picker.
2. Dip underwater in the mangrove swamp (be sure you are not actually in a river).
3. Find a regular swamp area and dip underwater there.

Expected result

Same shrinking fog over time in both biomes.

Observed result

Immediate full visibility in the mangrove swamp.