when a block next to it get a block update the mushroom mix with snow will only drop the snow balls and also plants aren't suppose to be in snow like that I'm petty Shure to.
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Also the snow layer that covers the mushroom is destroyed immediately as well even with a valid support block, relates to MCPE-187013.
Plants like 1 block tall flowers and small mushrooms are intended to be snowlogged in Bedrock Edition.