Ultegra FC-R8000 Right Crank Arm Creak

I have a 2018 Trek Domane SL6 Disc that I’ve put ~2500km on. I purchased an R8000 dual Stages power meter (Left and Right units) and fitted it onto my bike (following all the steps outlined in the dealers manual, and on many youtube videos). I still have lots of life left in my chainrings, so I swapped put my old chainrings on my new right crankarm. Since then I have been getting a creak when I put a decent amount of power into the pedals. My troubleshooting steps have been as follows (with the creak remaining after each one but the last):

• Checked the torque on the chainring bolts (installed to 14Nm)
• Cleaned the hell out of the drivetrain re-lubed
• Had the LBS replace the bottom bracket and re-install the cranks
• Checked the torque on seatpost (and then cleaned the seatpost, re-applied friction paste, and re-torqued)
• Checked the torque on the seatrails (and cleaned, applied a light grease between clamp and rails)
• Removed the rear derailleur, regreased, and reinstalled.
• Removed the pedals and tried riding with flat pedals.
• Put the new chainrings on the power meter cranks (and then swapped back once it didn’t help).
• Put the old (non powermeter) left crankarm back on (with the power meter right crank arm).
• Finally – I installed the old right crankarm with the old chainrings with the new left crankarm (with power) – and voila – no creak!

I then swapped the old chainrings back onto the new right crankarm and the creak returned.

Other than a manufacturing issue from Shimano, or a power meter installation issue from Stages – I can’t imagine what the mechanism is that would cause a creak from a right crank arm (given the above steps).

Has anyone experienced an Ultegra R8000 right crankarm being the ultimate culprit of a creak?