Network folders explains it. Collections cache the details of network files and don't monitor them for changes, since it could be problematic to monitor every folder containing collection items over the network. (For local folders, we can monitor the whole system with a small overhead, but there's no provision in Windows to do that over a network.)
Updating the cached values is probably going to be a matter or removing & re-adding the items. That can be automated for a whole collection using DOpusRT: External Manipulation of File Collections, or could be done for just selected files via a script. (Of course, a script could also be written which does both, and refreshes the whole collection if nothing is selected.) Time permitting, I might be able to look at writing that, although I can't right at this minute.