The expression 'hen dia dyoin' was not used by Greek grammarians, but it is frequent among Latin writers.
Why did it come into English usage in this corrupted form?
Can it be traced through English lexicons?
The OED’s earliest citation for hendiadys is from George Puttenham’s ‘The Art of Englishe Poesie’, published in 1589. The etymological note for the entry reads:
Google's Ngram Viewer shows only hendiadys showing up in English books, and it turns up around 1820:
Etymonline.com says hendiadys goes back further:
It also has no entries for hen dia dyoin or hendiadyoin.
Together, these things suggest that hendiadys is the term adopted into English rather than the origial Greek or Latin hen dia dyoin and hendiadyoin.
I don't have a definitive answer, but Anglicised loanwords often take on a different form from their original and drop accents. Note the Latinised form is hèn dià dyoîn. For example, French risque is first recorded as risk in 1728 and French bâton as baton in 1540s. There are many here and many more here.