I just noticed that the Grasshopper curve rendering line weight appear to be affected by Rhino layers Print Width when Print Preview is enabled. I can’t quite find a pattern, but here’s an example of this behaviour:
This is still an issue in
Version 6 SR17 (6.17.19218.13231, 08/06/2019). As far as I can tell, the Grasshopper curves will render with the width of whichever layer is first (i.e. highest up) that has a custom Print Width set.
Update: This behaviour does not occur when it Rendered/Arctic mode. Perhaps it is related to this bug.