And you are 100% sure that you are not in open mode (there are no warning screens when you boot up etc)? And you only used -f -F main.bin when flashing, as some arguments can trigger open mode? And how about when you change your security code from the settings UI?

And depending on MfE provider it can have some forced password policies, ie in some cases you must enter numbers+letters+special character, usually plain numbers are not enough, but you have to ask about those limitations from your MfE provider.