The TLfi has the pronunciation “/pʀopo/” and that’s the one I use, so there must be a non negligible proportion of the French population using it.

This discrepancy can be explained. The phonemes /o/ and /ɔ/ are both mid rounded vowels and have very similar pronunciations when not in the final syllable of a word. They have nowadays evolved to be pronounced the same in lots of places (there are still regional differences). That’s why whether transcribed as a close-mid /o/ or an open-mid /ɔ/ you hear the same sound in the files you’ve been listening to. You might also find the phonetic transcription [prOpo] using the archiphoneme /O/ which is used to transcribe both /o/ and /ɔ/ by some phoneticians.

I am pretty sure [propo] is the most common pronunciation in France nowadays, but it has not always been. Not being a specialist in the comparative historical phonetics of French I cannot tell when the change became perceptible nor in which French speaking parts of the world [prɔpo] is still prevalent.

The last syllables being stressed in French, the pronunciation of their vowels is not prone to change, so there has been no such evolution of the /o/ sound of the last syllable. But I suspect (only speculation) that this final /o/ has influenced the evolution of the preceding /ɔ/. What makes me say this is that a lot of people still pronounce /prɔpr/ (propre) although it is also a two-syllable word.