In my case I *do* get different behavior in emacs -Q between 26.3 and 27.0.60 (75a9eee8b). I had to replace minibuffer-keyboard-quit with abort-recursive-edit, since the former doesn't exist in 26.3. The initial minibuffer hint looks the same in both cases:
0
1
2
3
4
5
6
7
8
9
10
Test1:
^
|--- cursor
After pressing C-f, 26.3 gives the expected result:
^
|--- cursor
However, 27.0.60 gives me the following bizarre content in the minibuffer (where did that square bracket come from?):
1
2
3
4
in which the cursor is on the 0 at the very top. The read-key function appears to be identical between the two revisions, aside from a trivial change to support tab-bar, so I have no idea where the culprit may lie.