Ugly GRX RD failure...

[572cv]

Wife and I went for a fine ride yesterday in and around St-Émilion, France with pals, we on our own bikes, they were on rental ‘gravel bikes’ set up with GRX. Going up a slight hill, but maybe the best hill of the day, my buddy’s bike experienced catastrophic RD failure, illustrated in the pictures attached.

The pitch on the climb near the end of the ride of about 35miles led to an upshift for him. That had previously been ok, but at this juncture, there was a little skip to the shift, and then BOOM, the plastic covering on the upper hinge of the derailleur cage exploded, the chain jumped, then locked. The activity appears to have bent the cage. He was quick enough to release his shoe to keep from going down on his side, otherwise... who knows. He's had a lot of bad luck in the past few years.

Has anyone seen this before on a GRX set up, or any other? Any theories as to cause? He has to take this back to the rental guys and 'discuss' the situation with them. I was right behind him, and no way he did anything abnormal, experienced rider and all that. Also, if I was right behind him, he wasn't going fast!

Thanks for any observations. BTW, a ride picture to be posted in Photos from rides.