I have a R5 with 1.7.0 currently installed. I have downloaded the 1.8.1 update and placed it on a formatted CFExpress card. ( I'm on a Mac with Monterey 12.6.2) When I put the CFExpress card in the camera and go to the Firmware menu item it shows...
Yes, the 1.8.1 file is the only file on the freshly formatted CFExpress card. Like you, I don't think the modification date is the problem. I just found it a little interesting....
