This bug only seems to happen when Crimson Fungus Trees are grown very close to other ones, the Weeping Vines will wipe out a block (sometimes even multiple or all) of the stem blocks, considering the stem is the most important part of the block this makes farming a bit of a hassle.
Thank you for your report!
We're actually already tracking this issue in MC-174692, so I resolved and linked this ticket as a duplicate.
If you would like to add a vote and any extra information to the main ticket it would be appreciated.
If you haven't already, you might like to make use of the search feature in the future to see if the issue has already been reported.
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