Why is my email "currently unavailable" when I try to switch my Apple ID to that address?

I recently decided I wanted to change the email address associated with my Apple ID.

For sake of simplicity, let's refer to the email address that my ID is currently associated to as "email A" and the one I want to switch to as "email B".

I thought this would be simple enough until I realised that email B already had a separate Apple ID associated to it (we'll refer to my actual Apple ID that is in use as ID 1 and this "separate Apple ID" as ID 2). I knew that if I wanted to associate email B to ID 1, I needed to first disassociate email B from ID 2.

So I made a completely new third dummy email (let's call this email C). Using it, I changed the email associated to ID 2 from email B to email C. Now email B was free! Since email B wasn't being used by any Apple ID now, I figured I could change the email associated to ID 1 from email A to email B.

However, when I tried to do so the system wouldn't let me, claiming that email B "is currently unavailable". I'm sure that email B is no longer associated to any apple ID (I've even tried doing the whole "I forgot my apple id" process with email B but the system told me that email B WAS NOT an Apple ID, so the email itself must be in the clear) so I'm really confused as to why I can't switch.

Thank you in advance for your help and I'm sorry for this convoluted and confusing situation!