Agreed
Also agree that this could be made clearer.
However…
This part is totally on you. Even if you weren’t aware there was no joint account access before signing up, you are now.
You can certainly complain that there isn’t joint account access for Plus, but you can’t really complain about paying for it for 6months when it’s your own decision to keep doing so.
For the record I want Plus and Premium to be available for joint accounts, I have a joint account and want to upgrade. I’ve voted for it on here and Monzo will get my money when they provide what I want.