Disadvantage and advantage shall cancel each other out, even when there's different source of disadv

Yeah, this for sure needs to be fixed. A core rule of 5e is that even if you have 20 disadvantages, 1 advantage will cancel it to make it a straight roll.

It's actually by design, one disadvantage only cancels one advantage (and reverse), I personally like this idea it's more "realistic" and more tactical from my point of view to be able to counter a disadvantage.

Being "heavily obscured" and being "in darkness" should be the same, though.

Yeah, the main idea behind saying "one disadvantage/advantage cancels out any number of its opposites" is to avoid bugging down gameplay with tracking these sorts of things. With the aid of automation, that's no longer an issue. And yet it is, because you may realize you have disadvantage and want to cancel out the disadvantage but, before you do that, you need to manually count how many advantages and disadvantages apply to figure out if it's worth the while. 

I recall an encounter where everyone was fighting in magical darkness. As a result, all attacks were made at disadvantage. By raw, two blinded characters attack each other as per normal. This might seem seem weird to people, but it has it's purpose, When all attacks were at disadvantage, the fight really dragged out with tons of misses but, if I wanted to play optimally, I had to stay in the darkness because I was still allowed to target people with saving throws, which gave me an edge in a tough encounter. D&D assumes that creatures hit each other about 60% of the time because less than that is both not fun and drags things out. Having simple rules for advantage/disadvantage cancelling each other out helps assuring that disadvantage doesn't become too common and that regular bounded accuracy stays the norm.

Somebody stop me if I shouldn't discuss game features in the bug report forum ^^;

Well no. The main idea behind "one advantage/disadvantage" is so that things which are good are still good in all situations and so to not encourage effect stacking. Because having to debuff an enemy 5 times before they're effected is not fun.

If an enemy has two sources of disadvantage to your incoming attack then pushing them to the ground does nothing. The game wants to discourage this so that you still attempt to push enemies to the ground to get an edge. And also so that fights do not devolve into an advancing set of advantages and penalties.

If Solasta is not using that rule because it wants to keep track of this then it should not be. I do not want to have to worry about pre-buffing encounters because enemies will generate multiple sources of advantage. Its not fun.

It would be appreciated if Devs can clarify if this is intended design or bug. But by RAW, 20 disadvantage should still cancel out by one advantage.

It's one that gets homeruled at a lot of tables but you're right, RAW even 20 v 1 just cancels out.

In this circumstance obscurement due to darkness is definitely only one source of disadvantage.