To be honest, I think there is no real rhyme or reason* to CIS/DHS requiring biometrics for the I-751 even if they've already gotten biometrics for the I-485.
I had my biometrics done for the 485, I had to get biometrics done again for the 751 on February of this year... and as of today my CIS log-in account only says "Case was received." Go figure!
(* Actually, I do think there is a reason, I've noticed most people who need to get biometrics re-done come from the Global South like myself.)
And now apparently the soonest I can get an approval for the 751 is on July 2023, lol.
I did received the 24-month extension notice at least, but that only lasts until October 2023, so this is cutting it dangerously close.