Color Gradient Issue when Importing PDF into Inkscape

Question

I am making a figure with a colorbar in R, which I am importing into InkScape for annotations, etc. My workflow is to create the figure in R, save it as a PDF, and import that PDF into InkScape. However, when I import the PDF into InkScape, the colorbar of the figure gets changes from just a vertical gradient to a vertical + horizontal gradient.

To illustrate, here is the colorbar in the raw PDF file:

Here is what it looks like when I import it into InkScape:

The central axis looks correct, but the lateral gradient is incorrect.

If I export the image from R as a PNG file and then import it into InkScape using the same procedure, the colorbar imports correctly. Therefore, it seems that the issue is related to vectorized graphics. However, I would prefer to keep the image in vectorized format to preserve quality at different resolutions.

The rest of the plot (including the raster that uses the color gradient) looks normal upon import; the only problem is the colorbar.

Does anyone know how to preserve color gradients when importing a PDF into InkScape?

For reference, here is the code I am using to create the PDF in R:

pdf(paste0(path.fig, "Figure_HeatMaps_NoText.pdf"), width=(190/25.4), height=(190/25.4), useDingbats=F)
grid.arrange(
  grid.arrange(p.dd.years + theme(text=element_blank(), plot.margin=unit(c(0.5,8,4,0.5), "mm")), 
               p.dd.diff + theme(text=element_blank(), plot.margin=unit(c(0.5,8,4,0.5), "mm")), 
               ncol=2, widths=c(0.8, 1), newpage=F),
  grid.arrange(p.MaxTSA.years + theme(text=element_blank(), plot.margin=unit(c(4,8,0.5,0.5), "mm")), 
               p.MaxTSA.diff + theme(text=element_blank(), plot.margin=unit(c(4,8,0.5,0.5), "mm")), 
               ncol=2, widths=c(0.8, 1), newpage=F),
  ncol=1, newpage=F)
dev.off()

This question seems peripherally related; however, setting useDingbats=F in my PDF export does not fix the problem.