Auto-zero shouldn’t be able to cause that (though in general I would turn auto-zero off when using an SRM as I find them so stable that auto-zero is more likely to make the reading less accurate).
Sounds like something is wrong with the powermeter. It could just be the batteries running low. Mine started doing that and I sent it in for service (in Germany - I’m living in Denmark). Came back working fine and the note said they just put in new batteries and updated the firmware.
I have an SRM with a 520 too.
Are you doing a manual zero (you are off the bike) when you see the offset numbers go high? I could see auto zero causing the zero offset to go that high but then I would just get off the bike and do a manual zero and see what it is. If the zero offset returns to normal then yes it’s the auto zero causing it. If the number reads all over the place during the manual zero, its the pm.
I turn auto zero off. I had too many times where I’m soft pedaling into a stop light/stop sign and then I would stand up to accelerate which would cause my zero offset to read wrong and then my power numbers were way off.
Call SRM something’s wrong with your SRM. if it does not self correct when you coast for a few seconds you have something wrong. It’s similar to back pedaling with a Quarq. Like Steve said turn off off auto zero, tends to cause more problems. Auto zero works great on SRM head units, but Garmin head units at times can be funny.