I thought this was a problem with cura, and opened a ticket - but on cliser inspection it is gcode.ws that is getting confused.

When I sliced at 0.2mm, layer 75 came out in the viewer (and in octoprint) as having no gcode 9although looking inside the gcode, i see it).if I turn off 'sort layers by z' then visualisation works.

https://github.com/hudbrog/gCodeViewer/issues/53