I didn't look into your account too much, but of your last 10 games you have a 50% winrate so your MMR is now "normal" according to OP.GG it's 14 less.
At higher elos it fluctuates more heavily since MMR also considers how good players are around you.
For instance if you stop playing at D4 and everyone that was D5/D4 goes up one tier then you are considered D5 even though you haven't played. It happens faster the higher you get.
Since you seem to be able to easily get passed even Plat the system is slowing you down a bit as you are taking on a lot more losses in Diamond than in lower elos. Even if you lost only 5 matches before P1 losing 15 or more in D5+ means a lot more since it's 3 times your loss rate.
Of course I'm only saying this as a guess based on my similarities I experienced on my smurf. I'm having heavy LP gain losses in D5 down to ( 21 from 30 )as well despite having a 61% win rate games out of 89 games.
Or perhaps post D5 they just slow you down. I don't know, but it's not unique to your account is all I can say.
However, my losses are not more than my wins so that's very strange I do not believe you should be experiencing that unless you were on a loss streak which you are not.