A Little Child There is Ybore

So today you get an odd little thing that just worked itself onto my youtube suggestions. What intrigued me about it is that it has retained some Middle English spelling. And that eia exclamation is something I've come across before. I went down the rabbit hole last year, trying to track down the etymology when I posted Eia Martyr Stephane. But I had no idea what "susanni" meant.... And here is where things get confusing. Apparently, "susanni" (which has a great variety of spellings) is a German word. (I double checked this and found several other sources claiming the same thing.) There is a German carol that contains the "eia susanni" lines as well. The problem is, the English carol is older. It is attested in Ashmole Manuscript, dated 1393, and stored in the Bodleian Library at Oxford. Hymns and Carols of Christmas gives the lyrics to the English version, and gives comparison links to variants, including the Middle English Ashmole version, Gloria Tibi Domine. You will notice that the worlds correspond almost exactly with the more modern English of the carol I am posting, but with Latin refrains instead. Further searching was interesting, but didn't shed a huge amount of light on how we went from a macaronic carol, to one with an odd, borrowed German word, and that curious "eia" sticking out like a sore thumb. The nearest I can figure out is that the Germans borrowed it from us, sometime around the 1500s, and then we borrowed it back, retaining the "susanni" bit. I suppose that in this case, the "eia" could have been acquired the same way, but I am dubious of this. To start with, Eia Martyr Stephane dates to about the same perioud as this tune, so we know that it was an English expression. Secondly, most of the resources I found in my poking were far more concerned with explaining where "susannni" came from, and if they bother with "eia" at all, it is generally dismissed  as "an exclamation", with no effort to identify it as a particularly German exclamation. I am a very amature philologist, and ahve no business saying things like "my instinct says that it got into the Middle English version of Susanni independent of German influence"... but that is what my instinct says :-)