Dan, first of all, I note that the .ad4 file you sent today contains additional GPs over the one you sent last week. The file you sent last week does not exhibit the problem of backing texture only showing for those GP lines. I have attached the file you sent last week with the ARP adjusted into the N/W quadrant. It displays all GPs properly.
While I don't yet know what is the problem, I know a couple things it's not. It's not due to the ARP being moved. And it's not due to the z-bias of the underlay and the GP being only one digit apart. I do note that when I list the GPs in ADE, the original file does not show the associated material (which is OK) while the updated files displays the material for some of the GPs, suggesting they are new or have recently been edited. The display - or not - of the associated material is not a contributor to the current issue. However, I'm at a loss to explain why this is happening. It may be a "Pv4 thing", like the problem discovered last week, in which case there's nothing I can do about it other than, perhaps, developing a work-around.
I'm going to suggest you (re)start with the attached .ad4 file and re-implement your changes, testing after each change to isolate the GP causing the error. Once you've identified the culprit, please get back to me and I'll try to make some sense of this.
FYI, Jon, any GP that does not explicity state a material name should be tagged as using the "Default" material. Also, is there a technical reason why I can't run two copies of ADE side-by-side. I appreciate in normal circumstances this is unnecessary, but being able to do so would facilitate troubleshooting problems like this.,
Don