The bug

Frog spawn rates are low in the mangrove swamp.
The swamp update focuses on mangrove swamps, so mangrove swamps should not have fewer frogs than other biomes.

Steps to reproduce:

1. Find the mangrove swamp.
2. Use the following command:

   /tp @e[type=minecraft:frog,distance=..1000] @p

3. Found that only 10 or fewer frogs spawn within 1000 blocks of the player.

There are far more frogs in the swamp than in the mangrove swamp.