Just to note, most Israelis would say ענב /anav/ like the actor here, and would not think it's incorrect (for many years I thought the /enav/ or /eynav/ is just a more literary form. By some definition, that's just the case...
A line with /eynav/ from a classic song: https://youtu.be/5X_rBWAFfaQ?t=30.
The reason is really complicated, and I don't know well enough to be the one to authoritatively lay it out for you, but essentially the reason this, and SO MANY other words change pronunciation so extensively is because of how the vowels compensate for the addition of new syllables. Because of the nature of the vowels (I've been told they serve a grammatical function rather than a definitional function) they can often seem to go absolutely haywire under the right pressure. In this instance, when "ענב" becomes "ענבים," the tsere (which is an e vowel) attempts to shorten to a "vocal shewa," "vocal" being a fancy way of saying that it's pronounced. What complicates this process is the nature of the "ayin," or "ע," which cannot take a vocal shewa, so it compensates AGAIN by slightly UN-shortening the vowel into a hateph pathach. So basically it compensates for compensating, because the demands of the syllable conflict with the law of the letter (as opposed to the letter of the law? ;P ). Things like this happen to virtually every variation of every word. There's a rhyme and rhythm to it all, but I don't know it all nearly well enough to predicte how, when, why, and where. I don't know if Duolingo would be able to integrate at least some basic course about such fundementals, but it might be helpful for those with no experience at all regarding the vowel system, at least to gain a very rudimentary understanding that will help everyone stress less and anticipate more, even if they don't have the depth of understanding necessary to accurately predict every outcome.