I'm 90% sure it's a account-based bug. My account has had this issue for ~6 years now (I've used steam for 15 years). It happens on any browser or device. No cookie clearing, doesn't happen to any other account, etc. Every time I bring it up, the majority of people say they don't have an issue, while a small handful of others chime in about experiencing it too.

It has to be a bug, or maybe a security feature for accounts of a certain size?