The "solution" given by most rubricians

There is nothing to solve for rubricists. Authentic (current official) edition of the graduale is to be observed in chant, regardless of eventual textual discrepancies with the missal.

Thank you. It is helpful to know that the 1908 Graduale is the norm for chant, and to see Rossini's note about the discrepancies.

@Dixit_Dominus_44

Indeed, some official chant books even started to use the 'Bea Psalter' (approved under Pius XII) but that didn't last too long. *
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* There is disagreement as to whether the Pius XII psalter was ever mandatory.

I wrote my bachelor's thesis on the "Bea Psalter" (and the Neovulgate Psalter). It wasn't ever allowed to be used at Mass, the only exceptions being formularies created during Pius XII's pontificate (the new formulary for Assumption, reformed Holy Week). In Divine Office and rituale it hasn't ever been made mandatory, but since 1949 it was included in breviaries and other books as the default (and often only) Psalter version, so those willing to take the Vulgate option had to use an older breviary or purchase a supplement volume with Gallican Psalter.

@Dixit_Dominus_44 ad "Queenship of Mary":
ok, one more new (post-1945) Mass formulary I wasn't aware of, there may be even a few more. But concession to use the new Psalter version in "pre-existing" formularies, by, say, "updating" all the Psalm verses (psalm-toned and thus replaceable without breaking the music), has never been granted. (Anyone able to prove the opposite is most welcome to turn a good part of my thesis into ashes :) ). The widest concession ever, from 1947, allowed to use the Pianum "in all prayers, liturgical as well as non-liturgical, in recitation as well as song, (but only) of whole Psalms outside of the Mass" (AAS 1947, p. 508).

Ad Pianum being made mandatory:
let's put aside the ritual, which wasn't my primary object of interest. As for the Divine Office, interesting documents are the vota submitted in preparation of the Council: few demanded Pianum being made mandatory "at least in private recitation and in choir, when the Divine Office is being celebrated without chant", others demanded one Psalter version being made mandatory without opting for one specifically. Clearly, there was no one mandatory Psalter version at the time these vota were submitted, and sometimes the ambiguity was an issue.

To my knowledge, there is no act of liturgical legislation making Pianum mandatory. (Again, at least theoretically, a great occasion [edit: "opportunity" is the right word] to turn part of my thesis into ashes. :) ) 1945 Pius XII, in his MP "In cotidianis precibus", allowed to use Pianum in the Divine Office. At this time it was printed as a separate volume to be used together with the breviary. The 1947 SRC decree quoted above extended the concession to other liturgical and non-liturgical functions, under the conditions listed. 1949 editio nova typica of the breviary was promulgated, containing Psalms in the Pian version (only exception being psalm Venite, given in both Pian version and in the ancient one, with a rubric explaining that "in solemn chant the traditional text is to be used"). However, it's promulgation decree explicitly says that this Psalter version isn't mandatory yet (nondum). The decision to make it mandatory was never made. Instead, it was made (only) a de facto standard version for the Divine Office by being printed in the breviary as default.

(Sorry for the verbosity, "Psalterium Pianum" is one of my favourite topics.)

The FIUV position paper on the psalter lists the places in 1962 Missal/Gradual that use the Pian psalter (Appendix A).