In a snowy biome, when snow tries to form on top of a block it will instead delete the block it is trying to form on, replacing it with the snow layer. This means that if a player mines straight down and finds a place where bedrock is 1 thick and has no other blocks above it, he can wait for the single layer to be replaced by snow, break the snow and now have access to the void via survival means.

Biome used in the video: snowy_tundra, also tested in snowy_mountains, frozen_river, snowy_taiga, snowy_taiga_hills, snowy_beach, wooded_mountains, mountains, ice_spikes, deep_frozen_ocean, frozen_ocean, issue was present in all of them.

I have tested this on vanilla SP and vanilla server - this does not occur. I tested with and without plugins, it occurred in both instances.

video of issue occurring:  YouTube Link