The issue seems to occur much more frequently with Switft Sneak. When sneaking backwards and bridging, sometimes the player will fall through the block once placed. This has happened frequently in the nether when bridging over lava.

Here's one screen capture: https://photos.app.goo.gl/qiaLbmaLN6fj9NYm6
Here's another video: https://photos.app.goo.gl/1Tk2S7uqMM1K2kna9

Steps to Reproduce:

1. Equip Swift Sneak on leggings.
2. Set render distance to maximum (not sure if relevant).
3. Go into nether at lava level (32).
4. Start sneaking backwards and placing blocks rapidly as shown in the video. You will need to click for each block rather than clicking and holding due to another bug with double placing blocks (I've also reported that bug).

Observed Results:
At some point, the block you placed will delete and you will plummet into the lava, as seen in the video.

Expected Results
User should not fall into lava when sneaking and placing blocks. I haven't tasted in The End, but if it occurs there, this is very bad.