In Scottish criminal trials, the verdict can be guilty, not proven, or not guilty, and in this context the word "proven" is always pronounced "prohvun" and never "proovun". Is it pronounced "prohvun" across the entire legal field in Scotland, or is it sometimes pronounced "proovun" in that field?
This is pretty much just a matter of general pronunciation. Many Scottish people will make the "o" of "proven" sound like the one in "over", in all contexts. Others will use the long "o", which is an influence from southern English.
For the verdict, there is probably some conservatism due to the formal setting - and note that it is not a very common word anyway. So I think that a lot of people would say "not prohvun" even if they otherwise might default to "proovun" in an ordinary sentence. By the same token, someone with legal training speaking in a legal context might be more likely to say "prohvun", even if they would say "proovun" in everyday life.
It is not wrong to say "not proovun", because it's really just a matter of accent and everyone understands what is meant, but certainly less common.
For a real challenge, try "assoilzied".