I always check the manage my booking tool online from CX website, i never request for a seat nor have them assign one for me, im surprised sometimes they pre-assign seats usually a month before or more on some of my flights, and 90% of those flights with pre-assigned seats would mean an op-up for me. Anyone experience this pattern?

I experience the opposite...on "manage my booking," seats I have selected are never visible as an assigned seat. Instead, it has "--". The first few times I was worried, because I had specifically called MPC weeks before to assign myself a seat. But upon calling MPC back, they confirmed my seat assignments were precisely what I had booked with them, and when I flew it indeed was that seat.

i was pre-assigned a seat last time from HKG-DXB and CEB-HKG, i did not request for it at all, initially i was given a Y boarding pass but was upgraded at the gate. I now have a booking for CEB-HKG-CEB next month and i have been preassigned a seat for CEB-HKG even if i have not bought the ticket yet

That's odd - whenever I have requested a seat it ALWAYS shows up on my marco polo itinerary.

Assigned seats are always visible in my case. Occasionally they would show up after I log out and log back on. Once it showed up several hours later.

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 "--".

You can always ask MPC to email you the itinerary that shows seat assignments.