The bug
The Foxes spawned in a plains/river biome and not in the snowy taiga.
(Note from @unknown: there are a few blocks of snowy taiga in the corner of the chunk where they spawned.)
How to reproduce
Seed: 5065972378046467357
Coordinates: /execute in minecraft:overworld run tp @s -1368.57 64.00 -9713.01 217.95 16.80
Cannot reproduce in 20w30a. The provided coordinates teleports me to a luke warm ocean biome with the given seed. Please provide proper seed & coordinates.