Since legend league it’s hard to impossible to get back to past rankings.
I just submitted during the general restart due to new league and I couldn’t get to rank in the top 20 while I was usually ranked 2-3 with my current AI.
I noticed that Robostac who was rank 1 with 1-2 point higher than the 2nd resubmitted as well and couldn’t get back anywhere near his previous rank either, I guess that there must be something wrong with this new league…
I feel like there aren’t enough placement games being played due to the small size of the league (the same thing seemed true last week at the start of gold). A lot of the top bots are very close, but you only get one or maybe two chances against each, so while my bot might win 60% of the time, there is still a large chance it’ll lose any individual game.
On top of that the larger games are a lot harder to win (especially 3 player games, as it usually ends up with 2 bots competing for boxes while the 3rd wins), so the rank you get varies a lot each submission (eg my current submission has come 1st in all the 1v1 matches played)
I noticed that it is very quick to finish the submit indeed. The first 10-15 percents seems to account for about half of the total number of games you play to complete the submit. I suspect that there is just simply not enough game in regard to the ranking algorithm.
I see you managed to rank 2 now after a few submit ending up in between rank 5-16 that is a lot of variance, it’s pretty hard to work on improvement with such an unstable ranking 
Regarding those 3 players game I also feel they are quite unbalanced. If in the long run it should evens out, they are indeed more variance prone because of placement. Depending on the breakable box positions it can lead to 2 players struggling vs each other on one side while the remaining player kind of peacefully collect most of the other half, only entering the struggle in the late game.