My question is not simple to convey. Has the "genealogical community" such as it is, decided that grand-uncle is preferred and, if so, is there a stated basis for the claim? Because if this comes down to, well, one is in the OED from the 1470s and the other doesn't show up until a century later, that's a sillier argument that which-one-is-more-commonly-used. I am _not_ asking which one is correct. That's a question based on a false premise (that there is an over-arching frame which could provide a context for an answer).
I would treat this as an indication:
an uncle/aunt of one's father or mother. In U.S. frequently seen as "great-uncle" which is equivalent. Grand uncle is the older form, and is preferred."
This is _not_ where I first or second stumbled across the issue.