Weird bug involving OVER 1 (*solved*)

[einar]

It took me ages, but I finally managed to isolate this bug.

Here's a simple test program to reproduce this issue:

Code:

sub printA()
    PRINT AT 5,5;PAPER 6;INK 1;"A";
end sub

sub printBs()
    FOR f=9 TO 11
        PRINT INK 2;AT f,5;OVER 1;"B";
    NEXT f
end sub

INK 0: PAPER 0: CLS
printA()
printBs()

Both "A" and the first "B" will be printed with yellow background, the remaining 2 "B" will be printed with black background. However every "B" was supposed to have the same color!

Tested with several ZX BASIC versions (including latest version 1.4.0s1912), with different compiler optimization levels, on different ZX-Spectrum models. It never provided the correct result.