Question: Is it an issue with color values and XOR?
LINE (0, 400)-(1023, 415), _RGB32(0, 0, 0), BF
The color here is 255 Alpha(&HFF000000), so is NOT &H00000000, and that could be the source of your issue.
I'd suggest trying:
_DONTBLEND
LINE (0, 400)-(1023, 415), _RGBA32(0, 0, 0,0), BF
_BLEND
Clear that section of screen with a 0 RGBA (clear) box instead of a 0 RBG (black) color box.