I was able to reproduce something very similar, and I believe essentially the same issue perhaps varying due to config. I think I've actually seen similar glitches with python command line readline handling of <esc>k.

So, entering 1234 always works fine. The issue starts with 5 chars:

1234
12345
12345   12345

As you can see, the line is duplicated and going up to it shifts cursor to the right, at first the duplication on the right side is blank but if I move the cursor to the right, the '1234' are revealed. '5' is still hidden. Then I have to press 'a' to see the '5' revealed and go into insert mode.

There's no way to go to the left of the duplicated string.

I hope someone more experienced in readline or more motivated can look more into this.

I duplicated it in 3.11 built from source, on MacOS Big Sur (on M1 cpu, but that shouldn't matter).