The first time an event opens it's common that you get one of the best rewards because that's how tencent set up the rng, then since day 2 it goes to the normal average drop rate.

Is not oasis that modify it to make it less likely, it's just that day 1 is meant to be the day of the display for the new rewards (so they have by default a 100% drop rate).

That said here I'm jonin medal user, i do ultimate difficult basically every day and i continue to get once every two days one of the new good frags rewards (the 3 sannin youth, orochimaru akatsuki, edo jinchuriki and bee 8 tails chakra).

They simply updated the rewards to give a way higher bonus to ultimate difficulty because jonin medal became worthless after naruto froggy and they absolutely didn't want to apply the 20-50% ingots discount to every price in ingots jonin medal users in china have, so this was the compromise.

In comparison with before it simply increased the advantage of running it in comparison to not run it.