This has to be a regionalism because there're strictly identical to me, eg. in "Un train." /œ̃tʁɛ̃/ I say the two vowels exactly the same way.

After a cursory search it seems my Parisian-ish accent is at fault: https://fr.wiktionary.org/wiki/Annexe:Prononciation/fran%C3%...

Yup, very parisian. Love how then they almost mock how pain (bread) is pronounced in the south-west where you won't mistake the sounds between the words un pain.