The bug

When a tall(three-block tall or more) stalagmite(upward pointed dripstone) is attached to a block, and there is another stalagmite under the block. Then replace the block (with /setblock command) to a upward pointed dripstone, it should merge a taller stalagmite, but actually the stalagmite on the block and the replaced block will be broken.
In another case, a tall stalagmite is attached to blocks pillar, replace the blocks pillar(with /fill command) to stalagmite(keep one as the base), the stalagmite will be broken, only one upward pointed dripstone left as tip

This bug won't happen to stalactite(downward pointed dripstone).

In the Picture1, when you replace the gold block (with /setblock command) to a upward pointed dripstone, upper stalagmite and the new upward pointed dripstone(it's gold block before replaced) will be broken.
In the Picture2, when you replace the gold blocks (with /fill command) to upward pointed dripstones, under gold block will be replace a new upward pointed dripstone, other upward pointed dripstone (and upper gold block) will be broken
In the Picture3, When you replace the gold blocks (with /setblock or /fill command) to downward pointed dripstones, they will merge a longer stalactite in stead of be broken and fall down.