Whenever we have something like two ways to pluralize, we have to wonder the reason. The alternative declensions for rūpa and jhāna indicate to me that likely there was at one point instability between the boundaries of neuter and masculine nouns. Jhānā and rūpā are being pluralized according to the masculine grammatical gender in their alternative forms. I have no direct evidence for this, we'd need to see adjectives in the masculine corresponding to jhānā and rūpā etc., and even then a lack of those would only mean that the language regularized over time as written text. My model in this claim for a possible reason why there are two plurals is classical Arabic, which I studied for two years. Classical Arabic, i.e. "Korannic," is created out of several diverse Arabic languages, and has all sorts of irregular and stacked ways to conjugate and decline, all of them holdovers from diverse dialects that merged, like in Old Church Slavonic, but OCS is far more systematic and regular than classical Arabic and based on less languages.
The spotless mind,
the most highly pure, the tranquil,
all unspoiled phenomena supporting,
this name applying to the consciousness of the Tathāgata.
A bodhisattva, one of two vehicles, an ordinary person:
these are thrones which hold seeds subject to germination.
In acquiring the virtuous pure mind of a Buddha,
which is resolute suchness, the sūtra says:
The Tathāgata's spotless mind
is a pure place without outflows.
It is liberation from all bondage.
It is like a spherical mirror.
It is consciousness always in internal agreement.