I haven't had issues of old addresses reappearing (we've lived in the same house for 20 years), but I have had issues of the system changing the default address. Not for a few years though. We would often get stuff sent to Mr Tippy's old work, as the postie insists on safe dropping parcels here, even though it's not safe.

Mr Tippy had 5 months off work due to illness, so we started to get parcels sent to our home address again. I had reset the home address as the default address. Yet, every few purchases, the default address would change back to the work address. In the end, I had to remove the work address. It was the only way to stop it.

Knowing how glitchy eBay can be, it's entirely possible that this could still be happening for people that have more than one address in the system. Especially if they haven't removed an old address.