16 Wins In Row And Still Low LP Gaining

I ended the season in plat 5 and i was getting 20-21 lp per win , After patch 8.23 where 5th tiers of all elos were removed i became plat 4 but the lp gaining was different , i start getting 15-16 lp per win and losing 21 lp , I continued playing and i won 16 games from my last 17 games with two times (8 wins in row) , reaching plat 3 (91 LP) and still getting 16 and 17 lp and i'm playing vs plat 4 plat 3 players so how is this possible if my mmr is regular !! sorry for bad english
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