When I'm reading macronized texts, the prefix "pro" always seems to be marked long, with the exception of a few words in which it's followed by the letter "f": profugus, for example, and proficīscī, but not prōficere or prōferre. I've seen a handful of others that aren't coming to mind at the moment. Harm Pinkster's 2003 Latin-Dutch dictionary, which I believe corrects a lot of the vowel-length errors in earlier dictionaries (I'm looking at you, Lewis & Short), lists the "o"s in profugus and proficiscī as short. Ernout-Meillet isn't any help on this question.
Does this shortening follow a rule of which I'm unaware, or does it just, you know, happen sometimes?