Losing some transparency when saving to PDF

Issue already reported in the forum. Moving it here to follow it better.

There seem to be a problem when drawing multiple semitransparent scatterplots on the same canvas and saving them as PDF.

They look good when rendered with the GL GUI but the first one is somewhat flattened when rendered as PDF. Attaching the problematic code, a screenshot of the desired result and the pdf showing the issue.

Olivier Couet noticed that the issue might have something to do with the range of the histogram as it comes ang goes seemingly randomly just changing the DrawFrame ranges in the following code

auto c = new TCanvas("ctest", "", 600, 600);
// double x2=5; // not ok
double x2=4; // ok}}{{c->DrawFrame(-5,-4,x2,5);
ntu->SetMarkerStyle(20);
ntu->SetMarkerColorAlpha(kBlue, .2111);
ntu->Draw("c:d","","same");
c->SaveAs("testalpha.pdf");