There seems to be an issue with masking Power Fractals with unclamped colours, producing infinite displacement. In usual circumstances this allowed me to have terrain in valleys/canyons etc without the crease line, but in this instance it doesn't work correctly, only when masked through a surface layer.
In most cases masks are not clamped, so masks greater than 1 on your power fractal increased the amount of displacement.
The Surface Layer is more complex in terms of masking. "Mask as coverage", for example, results in a coverage value which cannot to above 1, so that might explain the difference between the two setups.
I suppose when masking canyons, the values at which I'm masking too probably aren't over 1 so much as this setup which is more sub-orbital and require such a tight clamp. Thanks for clarifying that, and it makes sense, and could be a handy modulator.