When pillaring up with any block, when you mine it down you will occasionally get bumped into a different X/Z as Y decreases. Typically it's about 0.5, so you may not immediately fall off until it happens a second time (and then you plummet to your potential death). This has happened since at least 1.14 and seems unchanged in 1.15. It is not related to server lag as it happens in single player as well as multiplayer.
This bug has been reported many times, but all of those have been closed with bogus reasons for resolution (when it is clearly not resolved).
Video from MC-237115 demonstrating the issue: https://youtu.be/niP4SJONmhQ
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What I am typically doing when this happens is chewing up ore blocks that were silk touched and converting them to their real pieces - I don't know if it matters that xp and such are at play here. I can reliably reproduce by:
Pillar up to build limit with coal blocks
Set yourself looking straight down, as close to the middle of the top block as you can manage
Using a diamond E5/F3/U3/Mending pick, hold down the left mouse button
It usually doesn't take long before you get slid over on the block you're falling onto (the camera never changes for me, unlike some of the other reports, so I don't know if that's indicative of something or not). If you were right in the middle of the block you probably won't fall, but if you keep going you will.

Also fwiw, this definitely happened in 1.14.4 - 1.15.1 doesn't change anything about the behaviour.
So, I tried to reproduce this. I artificially added server-side lag to my singleplayer world by setting randomTickSpeed extremely high and fell off the pillar a few times. However, I didn't actually die, but was teleported back up to the pillar. Is that also happening for you or do you actually fall down and die? (in 1.15.1, that is)

That is really interesting - I've never seen that particular behaviour. When I fall, I hit the ground (whether I die of course depends on normal factors).

Thanks for linking me to the main report. thought i would add what i know to the main bug report. acknowledged what has all ready been said, i cannot replicate this in single player mode either, nor with server hosted on my own network. the only server i can only replicate this issue on a server with high ping (300 ish) as the original post states mining directly down a pillar will push you off. i also remember this same problem happening in 1.14. i also have issues when mining a row of furnaces, my player position gets pushed back when walking into the space from the furnace that i just broke
The server i'm playing on had no server side lag it's just on the other side of the world
I am curious to know if pc specs have a part to play in regards to this bug being replicated in single player.
Im running a ryzen 7 1800X paired with a GTX 1070, the game/ world files are stored on an SSD
https://media.giphy.com/media/dUrY9lotCJz7brAKQk/giphy.gif

So, I got a new PC and can confirm at least anecdotally that system specs appear to matter. I have a Threadripper 3960X and this never happens in single player. It still happens on my laptop which is an i7-8565U (which is otherwise still plenty for playing minecraft).

This issue is still present even in 1.15.1 and it doesn't have to be in a 1x1 pillar. Currently, replacing walls with stone brick and my player gets pushed off of the wall as I mine straight down which can possibly kill anyone's character if the fall is high enough. The common denominator is mining down causing the player to get pushed off.

Cannot reproduce in 1.17.1 Release Candidate 1.

Cannot reproduce in 1.17.1.
This issue was resolved in 1.14.4 Pre-release 4 (see MC-156013), however it probably reappeared due to the fix of MC-156852.
You claim that this also happens if singleplayer. Can you provide a reliable way to reproduce this bug in singleplayer? I've been unable to do so.