Does the difficulty of explore affects the drop of rare traitstones (like arcane and celestial) or do they drop randomly?
Difficulty has no effect on trainstone drops.
It does. Mini bosses have a higher chance (up to guaranteed) chance to drop an arcane at higher level explores.
HOWEVER, you want to farm traitstones at the highest level explores you can one-shot everything. That will net you the most arcanes over a shorter period of time.
I tend to disagree. It does not.
Boss fight gives a guaranteed arcane regardless of difficulty (it’s just that doing Explore 1 takes more time to collect enough mythstones to unlock boss fight than Explore 12 would require); and while one could argue that mini-boss fight has a higher chance of dropping bigger traitstones than four preceding battles it is not, once again, tied to the actual explore difficulty (that is - Explore 1 to Explore 12).
At least, I haven’t noticed anything when playing my explores.
What Lorien means is Mythic Boss guarantee Arcane,
Higher lv let you meet mythic boss more often,
So more arcane.