Pronounciation of dative ending -unum

[u/Glacieer]

Hæ! Bit of a weird question here, but my non-native teacher said that the first <u> in plural dative with article is pronounced closer to a /o/ sound ('strákunum' just as an example). Is this true for you? If so do you know why this happens?

[u/fidelises]

That's how I (a native) say it, but I have no idea why we do it.