Technically, the system works exactly as you express you wanted it to. Unfortunately your MMR was so low that that even moving up its still bronze 5 level MMR. The reset algorithm works as follows (previous season's ending MMR+1200)/2 = reset MMR. Thus, winning 6 of 10 matches in placements would have netted you an approximate 50 point MMR increase meaning your MMR is now reset+50. if your previous season's ending MMR was, say, 500ish (not at all uncommon for bronze players no matter what the League System displayed ranking) your reset MMR would have been about 850. Add about 50 to that and you get 900 MMR, or still bronze 5 MMR. With the way the league system functions it is entirely possible to achieve a ranking displayed that is much higher than the MMR average of players at that ranking, again, not at all uncommon in bronze.
If you would, op.gg your account and hit the check MMR button and tell us what it says. I may be able to assist you further after you do so.