A hot issues bulletin went out about this recently, clarifying that registering online and at the machine are two separate things, and that adding a discount does not require online registration before it can be done.
Must have been very recent as I had a robust discussion with a member of staff at an Elizabeth Line station on the subject when trying to get a Priv discount added.
Given the requirement to add
any discount to an Oyster is that the "Registered" flag
on the card itself is set, just registering it online is pointless.
Shockingly, registering it online failed to set the flag in question.
I then visited a London Underground station and educated a member of staff there that it's possible to register a card without adding a discount at the same time, something they didn't know was possible.
Having done so, I visited another EL station and successfully had the discount added.