I created superflat world with Preset called Overworld, but added snow on top layer as follows:
minecraft:bedrock,59*minecraft:stone,3*minecraft:dirt,minecraft:grass_block,minecraft:snow;minecraft:plains;pillager_outpost,village,biome_1,decoration,stronghold,mineshaft,lake,lava_lake,dungeon
Mobs spawn in daylight inside lakes which are created under snow and get out of the lake and attack me. Also when I walk on snow, I can fall thru the snow into lake (including lava lakes), that's now cool.
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Duplicate of MC-610

Pardon me, but I don't believe it's the same issue, even if similar. Also severity cannot even compare - in one case you have few grass patches on sand, in this case you suddenly fall thru the snow into lava nethermind mobs in daylight? Should I make a video to illustrate a point or you can try to replicate it yourself?
I checked previous version - this is not an issue from 6 years ago, it was introduced in 1.14. Here are 4 screenshots, first from version 1.13.2, lakes look ok. Second is from 1.14, you can see mobs spawning in daylight. Third is lava lake under snow (inside view in spectator mode), and final is how this lava lake looks from above, you can walk right on the snow and fall into lava! Great!

With respect, I disagree. Other issues complain about insignificant visual appearance of few pieces of grass or snow, they say nothing about mobs spawning during the day or lava traps. This change introduced in 1.14 affects gameplay in a big way. Although I found this in superflat, I don't see why same issues cannot happen in default world in snowy biomes. If you still believe mobs spawning or lava traps are the same issue as few floating pieces of grass or snow, feel free to close this.