Confirmed in 1.16.1
Ok, a bit complicated but makes sense. Should have thought of that, thanks!
I just encountered this issue (1.15.2), and was completely lost as to why my commands weren't working before taking a deeper look into it. I set up five command blocks to teleport armor stands at one-block intervals along my sight. This screenshot was taken after having the command blocks teleport the stands along a line (wrongly starting at my feet), then running the exact command in the third command block (/execute at @p anchored eyes run tp @e[tag=3] ^ ^ ^3) which moved the middle stand up by about a block, where it should have been all along! (I also just tested with a funciton and in 1.6 pre-release 4 and had the same results)
Please take this bug seriously, as it is definitely messing things up for everyone!
[media]no crash report because it have to kill it in task manager
I agree that this is useful, especially for custom dimensions. But they really need to fix it so it actually works as one would hope. I noticed that the border size resets in dimensions other than the Overworld when you relog, and it's somehow split into two distinct pieces:
The visible part, which mirrors the border in the Overworld and stops player movement as well as block placement.
And the invisible part, which can be altered by the /worldborder command, causes damage when you cross it, and also stops block placement.
The most dangerous part about this in my opinion is that a player could pass the invisible border without realizing, until they start taking damage and can't interact with blocks that is.