Steps:
When jumping off from a high point, while in the process of mounting up (slidecasting mount up basically while walking off a ledge), you will fall down to the ground when the option is set to "Auto: Take off on jump".
If you set it on "Manual: Take off on double jump" you can still start flying midair, which is way nicer, you aren't forced to drop uncontrollable down and drop to 1HP then.

It doesn't matter if its gamepad or keyboard. PC/Console.

Both options should allow to initiate flying.