I want to make icc 25 heroic to farm the mount but it always sets it to 10 as I do to set it to 25
You probably have to toggle it back and forth a few times. I “farmed” Throne of the Four Winds for the mount for way too long before I realized it was defaulting to normal instead of heroic. I thought having all my toons set to mythic raid difficulty, it would default to heroic (as it would be the next highest difficulty). Nope. It went right to normal mode and my bad for not noticing it way sooner.
The mount from Al’akir can drop off both difficulties. Even back in Cata.