When bonemealing fungi, they can sometimes grow and replace existing blocks. This seems to be more common for non-solid blocks (such as redstone dust), but I have seen it happen with solid blocks (such as iron blocks).
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Thank you for your report!
We're actually already tracking this issue at MCPE-65661, so I will resolve and link this ticket as a duplicate.
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Also had this happen on the PS4.
1) Applied bone meal to crimson fungus on crimson nylium.
2) The resulting huge fungus replaced some (but not all) of the nearby nether brick fence with nether wart block.