I can’t speak to your issue of PowerTap hubs running low, but I think there are two other potential things that would explain the difference
First is you’re measuring total power at the hob and only left side power on the pedals. Thus if you have a slight leg imbalance that would explain some of the difference. This is not likely as you’d need to have a pretty severe imbalance, but it is worth mentioning since you’re switching from total power to left only
Second is drive chain losses. If you’re running a worn, poorly lubricated chain and haven’t thought about bottom bracket maintenance since spring then something approaching 10% difference is definitely possible.
Fortunately there is a relatively easy way to test these things! Unfortunately it requires a third power meter. Get access to another power meter and put that on your bike to compare. You could either throw your pedals on a buddy’s bike with a power meter and have them do a ride and compare the power readings from the two or throw another power meter on your bike to compare all three at once.
If you don’t have access to another power meter, or if the one you test against matches neither of the others, you could try replacing and optimizing your drive chain to see how close the two get to each other
All of this said - this isn’t much of an issue except in your brain. Pick which one you’re going to trust and use that moving forward. If you’ve got an 11 speed bike and can’t use the powertap anymore then who cares? Make sure the pedals are measuring consistently, reassess your FTP with them and move forward with your new values