Adding diagonal hints with the shortcut d can disable rasterizer preview. This does not happen every time. Glyphs 1313 on Catalina on 10.15.3
Also, doing other things can turn the preview on again. I’ve had deleting hints, switching to outline mode and back to TT mode, and changing glyphs all turn it back on. But none of them does it every time.
Adding and removing diagonal stems can enable and disable the rasterizer preview. I can open a glyph with diagonal instructions and the preview disappears, delete the stems and the preview reappears, and add the diagonal instructions again and the preview will disappear again.