As Automan stated above, if the box only has one set of hardware to decode the data from one of the tuners into usable data for the EPG download or Audio/Video streams, then once the box has tuned into the Freesat transponder and is decoding the EPG data, then it would not be possible to display the channel you were on in the background whilst this process is happening (unless perhaps the channel you were viewing was on the same transponder, but that would provide an even more inconsistent user experience.)
The fact that in the .17 firmware that pressing OK to schedule a recording pauses the EPG download and the Audio/Video resumes immediately I think adds weight to this theory. So if it's a hardware limitation, no amount of firmware changes are going to work this limitation during EPG download.