I would be surprised if your Alpe effort of 63+ minutes resulted in a zFTP worthy of B cat. That time generally equates to an effort of around 3.0w/kg.
You may have forgotten the big effort you put into your November FTP ramp test.
I can’t know your exact watts but I’m guessing you made around 30 seconds at the 340w effort. Your last 1 minute average would have been around 330w which would give an FTP of around 247w. I think an on screen message would have come up to that affect. That’s possibly why you think your FTP may be around 250w.
Anyway an FTP of 247-250 equates to around 3.5w/kg. However in itself an FTP of 3.5w/kg doesn’t always result in a zFTP (more important for racing) of >= 3.36w/kg to make it into category B. But of course it might have done.
In the last 5 minutes of your FTP there is a very good chance you put in an average effort of around 290w. The second part of your racing category is based on zMAP which Zwift deem to be your max 4-6 minute power. The 4-6 minute time varies for different people depending on their power curve however many hover around the 5 minute mark. 290w would give you a zMAP of 4.14 w/kg and for as long as your zMAP stays at your 5 minute power I think this will continue to keep you in Cat B.
Have a read of the first post in this linked thread. It will explain in a lot more detail and point you towards finding your own racing zFTP and zMAP figures.