This only occurs on the bottom row (screen scroll). Same error occurs if background set in a different viewport. You can see the loop runs consistently but changes behaviour when it gets to the screen scroll.
Code to replicate the error:
10 CLS
20 FOR I = 1 TO 6
30 VDU 17, 129
40 PRINT "RED BACKGROUND";
50 VDU 17, 128
60 FOR J = 1 TO 6 : PRINT CHR$(13);CHR$(10);: NEXT
70 PRINT "BLACK BACKGROUND";
80 FOR J = 1 TO 6 : PRINT CHR$(13);CHR$(10);: NEXT
90 NEXT
Produces the following: