So, i am pretty much at the 90 day mark now, and so far we've heard nothing about the Class B annotation.
We've been working through this process with Boundless Immigration and part of their service includes consultations with an immigration attorney. During one of these calls i asked about the annotation and she took all the information i had to offer and put my mind at ease. The lawyer said she'd be shocked if we ever heard back anything from it, it's highly unlikely anything will come of it, and that it's really not worth worrying about.
So, for any other folks who are going through this process and have found the same annotation on their K1 Visa, my journey looked like this:
Medical exam went fine, had to have 3 vaccines done to clear that part of the process. I have a short history with anxiety/depression, and the medical examiner required a care summary and full report by my designated GP, which i had to go away and get.
Following this the medical exam was filed, though i did not receive any confirmation from the office that it had been submitted to the embassy, i had to chase it up and contact the office to get that answer.
No comment was made about the medical exam at all while at the embassy.
Once i received my K1 Visa there was the class B annotation.
Arriving at Chicago O'Hare the officer took the big file with all my paperwork, made no comment about the annotation, though did confer with his colleague about it. I overheard the colleague saying "no that doesn't mean anything, just put it in the system and it's done".
The officer took all paperwork, stamped the Visa/passport, and i entered the country.
We got married 10 days later, and have heard nothing at all about the medical annotation on the Visa.
So, from my own experience i would say that if you too find this annotation on your Visa and you're worried about it, don't. It doesn't seem like a big deal in the process at all, nothing happened at the border, and nothing has happened since.