$3fff-graphics is shown when gfx display is switched on (char or bitmap mode), but of some other reason the normal rendering of gfx is disabled. Check out removing top and bottom borders or removing the sideborders. The opened areas (upper and lower border or the area between the opened sideborders) are displaying $3fff-gfx. Try setting some value to $3fff at the start of the code and see what happens.
When want to avoid showing $3fff gfx but still see it after setting $3fff to #$00 it might be so that you need to use character mode instead of bitmap mode (use #$1b in $d011 at $3fff gfx area).