Hey man! Think your content is great and the fact that you're interacting with people on reddit is awesome.
Some other people have mentioned it, but I think your grouping of Masters+ as one tier is misleading, because the skill difference between 0 LP Master and 1k LP Challenger is very significant.
Looking on u.gg, looks like Rumble jungle's Masters+ winrate is 47.71%. When you break it down between Masters, GM, and Chall, his winrate goes from 45.74% in Masters to 46.58% in GM to 51.45% in Chall. So his winrate isn't even dropping in higher skill brackets, the way you group the data just makes it seem like it is.
If you're looking at just 11.10 data, there are currently 49 recorded challenger Rumble jungle games. If you do 14 days, you see a pretty flat ~-3% win rate compared to the average jungler.
I'm interested in the sample being large enough that I'm confident in not having huge swings in win rate because of 1 result (keep in mind 55% win rate in 49 games is just winning 2 more than you lost and that's just unacceptable for this). Second, once I'm confident in low variance, I care about the trend. If a trend doesn't exist when travelling from bronze to diamond, why do you think one suddenly shows up when travelling from masters to challenger? It's possible for one to exist, but shouldn't be a leading theory.