When I learned that one says things like
Je voudrais quelque chose de beau
the rule that I either was told or generalized (I can't remember which) to account for the masculine adjective was that in this construction with de, there is no agreement.
However, in a comment on this question, when I wrote:
de sérieuse is impossible in any case (cp. « C'est quelque chose de beau »)
J'en voudrais une de sérieuse is correct French.
So what is going on? Did I internalize the rule incorrectly?
Is quelque chose somehow not feminine despite chose being feminine?
Is quelque chose feminine, but quelque chose de is a fixed genderless/masculine expression?
Some other explanation I haven't thought of?