Are you testing this while logged in as an admin or signup sheet manager? If so, and if you have the “Enable Sign-up form live search?” setting checked, then the fields will NOT be automatically filled in with your info. But, if you are logged in as admin, and you do NOT have that setting checked, your fields will be filled in automatically, BUT you can still edit them (NOT read only for admin and signup sheet managers so that they can signup other people easily).
I just tested here with latest versions of everything and can confirm when logged in as admin, it works as described above. When logged in as a subscriber, the fields are populated correctly and are correctly set as readonly, and I was not able to change those values.
Note that the very first time a user signs up for something, that meta data may not have been saved yet, so they may still have to fill out a form once. But, the next time it should all get populated and set to readonly.
It’s working correctly here for non-admin users the way you are wanting it to, with things setup as you described.