I have managed to re-create the same bug with different hardware synths. I used rev 2 and Nordlead 4. I can mention both of them works perfectly stable in Studio One 4.6
This is how I did...
My setup was totally stripped, no sound card, no nothing just one synth at the time. I also used the factory new song. (My synths were added both as keyboard and as instruments in the pref external devices. The synths did not use any aux channels just plain midi.
1. First out was my Rev2. The first midi channel (# 1) could record without problems But the freeze occurred when I duplicated the track and changed the midi channel (#2). As soon as I started to record midi data on this channel #2 -some notes and random controllers - the machine froze.
2. I got exacrtly the same result with my Nord Lead 4
It would be very interesting if you guys could try to re-create this too. Then we have a confirmed bug...at least most likely.