After living with this for 5 years, I have finally figured out the solution for the yellow plots. The root-cause of the yellow plots appears to be that the standard 256 color palette contained in a cluster coming out of an invoke node (FP.Get Image) become corrupted by the application builder on Mac OS X. The colors when running source code vs. when build into an application are very different. My workaround is to bundle the 256 colors directly in the source code. Of course, I am using 8-bit color. I would assume that if a higher bit-depth is needed, then a different set of reference colors would need to be bundled but I have not investigated that.
I also suspect the yellow icons are similarly corrupted but I don't know how or if I could solve that. But at least now I can save my plots using LabVIEW 2020. I hope this helps someone else that has been frustrated with this. A screenshot of the work-around is below. I also have attached a screenshot showing the yellow icon.