difference in meaning

Their meaning might depend upon context.

I'm going to Japan next month. I hear the Japanese are a hardworking people. Their babies are cute!

I heard the Japanese are a hardworking people from a guy in a bar called Mr. Sato.

I heard the Japanese were a hardworking people, but when I went to Osaka it took six days for my watch to be repaired!

Hard-working is hyphenated more often in AmE than BrE. Whether it "should" be is a matter for you to decide.

I think, as your dictionaries suggest, that it is optional whether to hyphenate hard-working.
I prefer it with the hyphen.

I would be wary of any guys called Mrs Sato I might meet in a bar. :wink:

Different strokes...