Rear Di2 derailleur lost tension, spring destroyed

Part: Shimano XT Di2 12-speed rear derailleur (RD-M8150-12), 250 hrs of use.

I removed the unit to clean. Didn't move the hanger through more than half of its normal range during the process. Then reattached, plugged in, and turned on the bike. When it powered up, it suddently lost all of the (chain) tension. I've done the exact same process previously with no issue.

You can see the tensioning spring extruding from the hole were it attaches.

Is there any hope to repair this? LBS guy said no and it's $450 to replace.

I would also love to know what caused the failure.