Plains biome is always generated when a swamp borders a desert, snowy tundra or snowy taiga

If the wiki is correct then this is normal.

I have seen this happen, though I have heard that 1.7 terrain generation is difficult to modify. Based on the resolution of MC-58393, this is probably either won't fix or intentional.

@unknown, river not frozen in Snowy Taigas or Ice Spikes is now tracked at MC-138801

Changing things like this would, in many worlds, definitely lead to sharp chunk borders between old and new generation, which as far as I am aware is something Mojang Studios wants to avoid.

Please note that, while the Wiki does offer a nice compendium of the game, it is not a valid source that a feature is Intended.

Can confirm in 1.17-pre5.

This appears to be fixed in 21w37a tested with seed: -5234808643815365891 where a swamp generated at spawn next to a desert but no plains biome bordered it/went in between the two biomes