I tried this out for interest sake and can't slidecast into a jump to takeoff with the auto setting. It just performs a regular jump when you mount midair, as expected. Got to press jump again to take off.

Maybe they could combine auto to include manual's controls and register a jump press to initiate flight. Manual seems to detect if you are falling while mounted, even of you slidecast off a cliff.

Or just use manual. It's a QoL thing not a bug.