Displacement is not visible in reflections/refractions

Displacement in the default "Screen size (px)" mode may not be visible in reflections or refractions if the reflected/refracted area is outside of camera frustum. 

The solution is to change displacement calculation mode in Render Setup > Performance > Performance Settings > Displacement to "World size (units)". 

Alternatively, you can use the following string option, however it may increase RAM usage:

float geometry.displace.maxSizeScreenOutFrustumMultiplier = 1