The Bug:

You cannot replace blocks supporting stalactites without the stalactites falling.

This issue is exclusive to stalactites (pointed dripstone facing downwards) and cannot be reproduced with stalagmites (pointed dripstone facing upwards). Please also note that if the entire stalactite consists of a maximum of one block and doesn't have air below it, the block that's supporting the stalactite can be successfully replaced without the stalactite falling. This is the only exception to this problem.

Steps to Reproduce:

1. Construct a stalactite of any size.
2. Switch into creative mode and attempt to replace the block that's supporting the stalactite by pressing the "Attack/Destroy" and "Use Item/Place Block" buttons simultaneously.
3. Watch the stalactite closely as it supporting block is replaced.
4. Take note as to whether or not you can replace blocks supporting stalactites without the stalactites falling.

Observed Behavior:

You cannot replace blocks supporting stalactites without the stalactites falling.

Expected Behavior:

You would be able to replace blocks supporting stalactites without the stalactites falling.