No it does not effect the resistance (it changes the element to the usual weakness), but the usual weakness does! Therefore, Effective does not increase the damage by 25% - that is definitely wrong!
If it would work like that, my observations would have been different.
This is exactly what happens. The interaction with Weaken is consistent with -25% monster resistance. Otherwise the damage values would be different with Weaken+Effective together.
600*1.25 would be 750, not 904.
Edit: although I still don’t know what happens if the “weak” resistance gets resistance because it is an epic monster.
Oh, and while you’re at reporting bugs :
Equality is bugged somehow, and Skilled Mythic does not show in the damage screen.