No mystery here. Dragons rate is terrible despite their "lvl100 dragon rate increased" and gold/rainbow rates on gacha are false too. Far above what they're supposed to be.
Also, a little adice on Nutaku. When you play it, just open another tab of anything aside it. And then, run your map. If you got
bad chests or fail RNG, just close the game (but not your browser) before reaching the end flag and you will not lose stamina. You can do it one or two times in a certain amount of time. It work on Google Chrome. never tested on other browsers but I guess it works too.
It's clearly cheating but hey, it's Nutaku.