GPS rescue landing question

I went flying yesterday with the greatest of all time @mstc.
We wanted to test some gps units and do a little acro in a new place.
Gps rescue generally seemed to work well, but @mstc was telling me that, in theory, if I don't manually turn off rescue mode and regain control, the quad should land itself. We decided to give it a whirl and I flew my crux35 with m10 gps out and then triggered the rescue switch. Quad came back and started to descend, but began some really wild up and down oscillations as it got close to the ground. Like it descended rapidly to within 10 meters of the ground and then shot up 5 meters and then dropped another 10 meters and then rose again. At that point I lost faith and manually took back control. I was clueless as to whether this was normal landing behavior but @mstc assured me it was not.
Any ideas what my quad was doing with the vigorous up and down? Any clues what I might need to adjust to get a reasonably safe self landing?
If I can remember to I will post video later.
Thanks all!

And just another note, quad is currently on betaflight 4.4.3