Is there a bug with dungeon difficulty rewards?
I play lv2 difficulty objective,and i reach over area 4780,but i only get 872 dungeon point and 7 dungeon coin.Then i play another lv1 objective,i just reach area 2801,and i already get 1020 dungeon point and 10 dungeon coin.Those two objectives are both dungeon point bonus objective.
Should't higher lv difficulty have better rewards?

I'm gonna have to agree with the OP here. While I haven't had the SAME issue as them in the sense of receiving significantly less rewards on higher difficulties, look at the following link. https://imgur.com/a/GJqLLTO The top two are two attempts at the same objective with the same formation with the same objective buff on two different difficulties. At level 2, I can reach area 13,275 for 9,310 points, and at level 3 I can reach area 9,600 for 6,050 points. If we divide the area reached by the points received...

13,275/9310 = 1.425886143931257 average points per area
9,600/6,050 = 1.586776859504132 average points per area

The ratio is borderline identical. A 10% increase doesn't feel very rewarding for playing on a higher difficulty, especially considering just how much more difficult each level becomes.

I also tested the rate of the idol buff with the second two pictures in that link. Level 2 is 13,209 for 3.17% and level 3 is 9,535 for 1.64%.

13,209/3.17 = 4166.876971608833 average areas per 1% idols
09,535/1.64 = 5814.024390243902 average areas per 1% idols

So it actually takes MORE areas to get the same payout on a higher difficulty. This may simply be because I only got to area 9600, not too much further beyond the 7500 Erika mentioned, but if you look at the area I reached in level 2, I made it WELL beyond area 10,000 in level 2 to unlock the next difficulty. If I'm able to reach a healthy 3000 areas beyond the requirement to unlock the next difficulty up, then why am I still not strong enough to reach deep enough in level 3 for the ratio to begin evening out? Let alone actually being an abject IMPROVEMENT since, y'know, higher difficulty.

Currently, it seems like the only justification for playing on higher difficulties is if the player can reach area 17,500 on that difficulty, because otherwise lower difficulties offer better ratios.

Fixing the point payout is easy. Just make the buff scale with difficulty. Level 1 is 150%, level 2 is 300%, level 3 is 450%. That way, unlocking higher difficulties actually feels like a notable improvement.

Fixing the idol payout is a bit more difficult because while raising the idol cap is fine, it doesn't actually MEAN anything if the player can't REACH that cap. And while the RATE might scale faster, it means nothing if the player can't get far enough for the ratio to balance out and compare to lower difficulties. As shown above, even being able to comfortably coast past the level 2 requirements for level 3, level 3 is still far too difficult to get deep enough to make it feel even EQUAL, let alone rewarding, compared to level 2. So the issue might be in either the difficulty scaling of areas or just increasing the rate that the bonus itself scales. The big issue with how the scaling currently works is that even if level 2 and level 3 both scaled to have IDENTICAL ratios, level 2 would still be better because the higher difficulty of level 3 means shorter runs and more resets per week. For the same ratio of payout, all that amounts to is burning weekly challenge runs much faster for no actual gain. The "shorter runs means more runs overall" debate only holds SOME water if there were infinite runs. Sure, if the ratio stayed the same across both levels and I was able to go from 100 runs to area 10,000 to doing 150 runs to area 7,500 due to the higher difficulty making more, shorter runs, that would amount to 125,000 more areas overall, meaning 12.5% more areas to apply that ratio to, but again, that's not much of an increase for what SHOULD feel like a more rewarding system, AND that model, again, burns weekly challenge runs - and therefor, tokens.

It just doesn't feel like level 3 runs are worth it right now. The drawbacks outweigh the rewards.

as the blog post on the subject said the rewards for diff2 are better if you can hit 10k and for diff3 are better at 7500. While they could (and arguably should) have given the higher difficulties diff1 rewards up to those points so they wouldn't have been a potential trap they didn't.

Idol bonus is a bit more complicated but a rule of thumb would be e3k or under damage do diff1, e3k-3.7k then do diff2, e3.7k to the max (area 12,500 for idols) do diff 3