According to an article I read today (in Romansh; possibly, specifically Vallader), Christ's ascension into heaven was referred to differently in the Roman and the Gallic (France, Spain, Portugal) traditions: as ASCENSA in the Roman (until C6) and as ASCENSIO (acc. ASCENSIONEM) in the Gallic (C4–C9).
However, by C10, the Roman liturgy had also started using ASCENSIO, and so words deriving from ASCENSA are now typically found only in dialects (the article mentions Italian, which has Ascensione in the standard language and Ascensa in some dialects/regional languages and in Old Italian, e.g. Dolomitan (as)senza, Furlan assense, northern Italian dialects (as)sensa).
However, Romansh preserved the older Roman-liturgy form even in the standard language, throughout the entire territory of the language.
Yet today, we find two forms: one deriving from ASCENSIO, one from ASCENSA.
Apparently, what happened was that when the Reformation came along, it brought the "newer" word, and that caught on especially in Engadine (where the Reformation was most successful), so now we have Ladin Ascensiun vs. central and Surselvan Anzeinzas.
And the article notes at the end that "To close, we can find that the Romansh Catholic regions are the only ones in Europe to have preserved the old form from the Roman liturgy not only as a dialectal form, but as the official form of the liturgical language and of today's literary language".