There is a reason why this happens 
The way Faster Payments work is when we initiate a transfer to another bank, they send us a response to say yes/no to the transfer. Only at that point does money actually move from your account and into our settlement account, ready for the other bank to collect at the next clearing cycle.
What that means is that when you are presented with the category chooser, the transaction hasn’t actually been completed - money hasn’t changed hands and therefore the category you select isn’t actually saved, because at that point there is nothing to save it against.
Only when you receive the push notification has your transfer been accepted by the other bank and therefore a transaction created 
So, it’s more a missing/broken feature than a bug in the traditional sense but as @Jim1701 says, it is being fixed!