It's common when the air spring develops a leak it will be in a fold and hold air just fine until some magical amount of suspension travel. Experienced the same with wife's car recently.

Lift the suspected corner off the ground to extend the spring and spray it with soapy water or some premix all purpose cleaner and you should see foam develop on one of the seams.

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2011 E70 • N55 (me)
2012 E70 • N63 (wife)