I'm not sure why, it sounds like its indigenous to my profile. For example, I have a round-trip personal trip to JFK in May. Normally my colleague does the seat assignment for my trips, but because this trip isn't for work I called MPC myself and got 11A both going and returning. However, on my "manage my booking" profile both flights come up as "--". Worried I may have lost my seats, I called MPC back myself a few weeks later and asked if I had seat assignments. The MPC agent promptly told me I had 11A both going and returning, same as before. I actually had this happen to me a few months ago as well on a HKG-SFO round trip, I wanted seats 2K on both flights and hence called MPC myself (forgot to assign seats when booking). Looked at my profile later, and sure enough "--".