pm2023 that's interesting - if 300x600 has never been enabled on the site then it would rule out caching. It's difficult for us to say when we can't replicate this so I'd recommend if you now have 300x600 deselected that next time you see this issue, send us the URL and a screenshot of the 300 x 600 showing and we can take a look.
It's worth mentioning though that the 300x600 is a sizing option, meaning it wont show to all users but will only show when our AI has gathered data on user behavior that tells us this ad would be preferable for this user. Having the 300x600 size as an option can triple mobile revenue too, so it's better to have this enabled.
Regarding your Premium question, I'm seeing that your last invoice was paid from your earnings - you can see a breakdown under account details in the premium dashboard: https://premium.ezoic.com/premium/account