When discussing temperatures or decades, should it be hyphenated?
I understood that two-word adjectives need to be hyphenated, but why does MS Word think this should be, too?
My American Heritage Dictionary has entries for the adjective 'mid' and the prefix 'mid-'.
Modifying the noun 80s with the adjective mid would be written as mid 80s.
But creating word by appending the noun 80s to the prefix mid would be written as mid-80s.
So, it depends on which form of mid you use.
Please note that if there is a standard dictionary for the project you are working on, you must ensure that there is an entry for the adjective mid, not just the prefix. If no such entry exists, you're stuck with the prefix mid, which means you're also stuck with the hyphen.
This is really a kind of corner case where there's probably no consensually accepted solution used and preferred by the majority of speakers.
I would normally go for the non-hyphenated case.
NB "mid '80s" is more correct for decades because it is a contraction of "mid 1980s" (presumably).