The bug

Free hanging corner vines are not correctly updated when the the block the vines above are supported by is removed.

See also Bug Of The Week 1: Vines in 1.12

How to reproduce

Use these commands

/setblock ~1 ~3 ~1 stone
/setblock ~2 ~3 ~2 stone
/fill ~2 ~3 ~1 ~2 ~ ~1 vine south=true,west=true
/setblock ~1 ~3 ~1 air