Another vote for dia-beet-ease. I’ve never heard it pronounced dia-beet-iss.
I’m from New Zealand.
The “iss” pronunciation has always struck me as being both:
(1) Old timey, and
(2) Rural
Kind of like “rillator” for “realtor.”
My aunt says Huh-WY-Yuh. It drives me bonkers. My grandmother used to say Io-Way instead of Iowa.