I don't know how often the Humax box refreshes the displayed guide - there is a possibility the guide is frozen when you are looking at it. So you wouldn't have seen a change, even if one was transmitted.
The guide is constantly updated by data transmitted along with the channel you are watching (Freesat does the same). The entire epg is in effect a round robin of data so it may take a little time for a schedule change to hit your epg. Programme overruns where recording is already in progress shouldn't need the epg to change, all it needs is the broadcaster to delay the change of running status to the appropriate time. If your box isn't set to AR then of course it won't be looking for this data. Do a factory reset and immediately call up the epg. You can see the process in action.
