When villages generate in an adjacent biome to mangrove swamps, and continue into the mangrove swamp, the villagers are still the same variant as the originating biome.

This is inconsistent with how villagers generated in the swamp will become the swamp variant, when a village generates partially into a normal swamp biome.

Villagers spawned in with a spawn egg in a mangrove swamp, however, will properly become a swamp variant villager.

For ease of reproducing the issue:
To go to a plains village that has partially generated into the mangrove swamp, use seed 20220518 and run the command: /tp @s 2745 105 1640 45 35

Image above:  Left villager was spawned in with a spawn egg, right villager was naturally generated with the village.