You didn't mention the antenna brand or model, and that would help.
It's a Diamond SG7500NMO. It's factory pre-tuned and has performed flawlessly in the past. Incidentally, I did try swapping it out with a 1/4 wave 2m whip, which also showed the same results.
As for the coax. The best length is the one that stretches from the antenna to the back of the transceiver. Any amount of extra length just adds loss especially at UHF. You should check the coax connections obviously.
Point taken. I will check the connector, but if the connector is bad, wouldn't it be bad on both 2m and 70cm?
Lastly, what are you using to measure the SWR, and where in the line is it?
I'm using a small, portable VHF/UHF SWR meter attached at the transceiver.