Well, you may be right about the bug, but I just don't have too much time to worry about this now, hence my 'shrugging off'. I still think if you mask a 'to be smoothed' area by small detail, you're bound to get weird artifacts. Not in the front, because the initial displacement is tiny or absent, but as soon as terrain goes up or down, these mismatches with computed terrain may play havoc (because of the mask).
Too smooth out certain smaller displacements like you have in front, I would always use an XYZ shader after the big displacements to 'fix' the big ground. The compute terrain has a certain patch size to consider (and slows down), hence my choice. Any added smaller displacement after that can be smoothed by whatever, and not affect the mountains over yonder. Maybe that's a 'workaround'?