1) Triplanar is by definition supposed to work outside of any UVW restrictions
I don't know technical details, but isn't this just a matter of order of operations? If displacement would be applied last instead of first, then it should give better result.
2) If we would "fix" that, then people would complain that they are displacing some landscape, and then triplanar is not working how they would expect :)
Trying to thing logically, how many cases there is, where current behaviour makes conventional materials unusable, versus number of cases where somebody wants to use triplanar for displacing the mountains? I think that Juraj and others already threw in some good ideas, how this can be improved. I think that this definitely should be
fixed.