Yeah, mangled checkerboard patterns are common when prompted to "remove" the background. It can be worked around by generating multiple images with only the background color varying (e.g. black and white) and reconstructing the alpha channel from their difference, as the model generally prefers to just copy and paste when no other prompts override that preference.

“Just do more manual work and waste even more energy so you can take yet another manual step and finally get what you wanted.” A real time-saver, that.