The bug I got basically happens because the store registers the attempt to buy a Series X as successful even if - for some reason - the purchase isn't actually completed, and the store blocks your account from buying "another" SX for 30 days. It's unknown why the purchase can't be completed, since the store doesn't specify the reason.
Apparently this bug was fixed, twice (which implies that it wasn't fixed at all...) and when I tried to buy the console again, for the third time, with the support doing the actual attempt, a new error appeared saying "insufficient funds"... Which wasn't true at all, since I had 506€ in my MS account, 375€ of which are Rewards funds.
I know, in fact the original expiry date has passed twice already, and the new one is set for the 29th of July... :/
I don't know if the preorder trick works, never heard of it before. I know of this guy who tried to buy the Halo limited ed. SX with Rewards funds and when his purchase got cancelled for no reason, he had to press the support to get the R. funds back.
Anyway, if the expiry date won't be changed again, apparently my funds will be available for one year even if I can't see them in my account, and when I want to spend them I have to contact the support. Not sure if true, but it doesn't sound very convenient.