No idea, but now that you mention it, it's the same for the Boeing registrations. No B-20B* ...
And... There's no B-30*B either So it looks like B has been banned from the Chinese registration except the prefix?
Is it do with B being important as in Beijing, so you don't then use a B in the minor part of the registration ?
It sort of reminds me of working for Honda when we refitted the reception with nice red seats, red seats to represent the red on the Japanese flag. Then we found out that they would not sit on red seats as it was disrespectful to the flag !