Thile generating some dummy passwords for yhis I just noticed that

qweqweqwe - works
QWEQWEQWE - does not work
123456789 - does not work
12345678 - does not work
12345 - does not work

I remembered that my working longer than 8 cahracters password had single uppercase letter at the end. Then I started to suspect the input method and I think I found a reason.

When multiple "special" characters are entered it is convinient to HOLD DOWN the shift or alt key while typing then just like in X Terminal or everywhere else.

Boot menu does this differently, the shift/alt,ctrl keys are for single use only, so to type 12345 you have to press-and-release alt,1,alt,2,alt,3,alt,4,alt,5,enter. Why ?

If the typed characters have been briefly echoed, this had been easier to find out.