When there's a small portion of lava and you place snow on the lava, the snow doesn't melt. it does melt when there's a pool of lava, but even with a small portion of lava, the snow should always melt when interacting with lava.
How to reproduce
Start by obtaining a lava and snow bucket
Then place a small portion of lava on a small hole like shown in the video, then place the snow on the lava and it will not melt
Linked issues
duplicates
Attachments
Comments

Even fluids can replace other fluids:
[media]
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We're tracking this issue in MC-8930, so this ticket is being resolved and linked as a duplicate.
That ticket has already been resolved as working as intended, which means this is not considered a bug and won't be fixed. Please do not leave a comment on the linked ticket.
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To be honest, I don't see why it's an issue. Powder snow is a block instead of fluid, and blocks can replace fluids.