The arp. pattern is not saved in the multi but in the single program. The only thing saved in the multi is which singles is used in each part, the delay/reverb settings, the MIDI channel used in each part, the keyrange for each part, the volume and pan of each part, the output assigned for each part, detune and transpose for each part, and maybe some settings i forgot. All sound information (except the global delay/reverb setting) is saved in the single program. You should read the part "all about the memory" (page 47), "multi mode parameters" (page 110) and "the virus and sequensers" (page 140) in the manual (and read the rest of the manual too
). I can´t explain what happened in your case...