EDIT: Disregard the entire post cuz u/PCBS01 managed to prove that Mortenax's E2 does straight up make Ultimates also be considered FuAs, therefore it does synergize with Ashveil among other things. Check their replies below for more details (It still doesn't make Ultimate DMG also be considered FuA DMG though, which is a separate distinction)
So this may or may not be important, but I noticed that Mort's E2 says that when an ally uses their Ultimate to deal damage, it's considered as launching a FuA:
Meanwhile, the Feixiao Trace that turns her Ultimate into a FuA doesn't say that it counts as launching one, it's simply considered a FuA:
What does this mean? Well for one, some people pointed out that Acheron's E6 doesn't specifically make her Skill be considered an Ultimate and thus it wouldn't be a FuA with Mort's E2
But I think this might have some other implications that not even Hoyo themselves intended, which is that Mort's E2 might not even be making Ultimate attacks benefit from buffs to FuA damage at all.
To explain further, while Feixiao's trace's wording directly makes her Ultimate be a FuA, Mort's E2's wording instead makes Ally damaging Ultimates count as launching a FuA, which is more related to the action of triggering a FuA, rather than the attack itself being one (Think of how Feixiao's Skill itself isn't a FuA, but it launches one afterwards)
And if that's true... Then it means Mort's E2 wouldn't work with Ashveil's Trace, and would instead work with Feixiao's E2, which does specifically include the wording of launching a FuA:
Mind you, if all of this is true, I think this would be completely unintended by Hoyo, given that it would mean Mort's E2 synergizes with an old unit while it doesn't synergize with a newer one, and as we all know that's pretty much the exact opposite of what they typically do
At the same time it could easily just be a translation error and Mort's E2 does work exactly like Feixiao's Trace and they'll end up changing the wording to be the same as hers, after all it's also not uncommon for Hoyo to make mistakes by wording things differently than how they actually work
But yeah, this is solved as easily as someone testing this to see if Mort's E2 makes damaging Ultimates be buffed by Ashveil's Trace or not, but unfortunately I don't have access to the Beta so I can't check for myself