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