With the settings you described the M Maximizer should NOT let peaks hit the 0 dB ceiling, I know for sure it doesn't on my side.
Are you using the maximizer as an insert effect in the master section ? if yes, make sure the master fader is set to 0 dB or below. The master fader always acts after the insert effect, so pushing it above 0 dB will inevitably cancel out any brickwall limiting you applied.
Another possible cause could be soft clipping simply being turned on, it can introduce audible clipping with high knob values and it's normally unnecessary with look-ahead activated + fast attack.