QTextDocument renders html with multiple font sizes differently on sequential runs

When drawing a QTextDocument containing html text with a mix of font sizes to an image a subsequent draw of the same document results in a different output.

From testing it appears to only fail for some font sizes (though does fail for both pixel and point specified, and the order the sizes are used also appears to be significant.

e.g. 

"<span style='font-size:100px'>100 px font</span> std"

and

"<span style='font-size:100pt'>100 pt font</span> std "

fail, but

"std <span style='font-size:100px'> 100 px font</span>"

works, as does

"std <span style='font-size:100px'> 100 px font</span> std"

It also fails if you create two QTextDocuments containing the same text in the same way. 

For the attached code on my local ubuntu xenial system I get the output and the included images (with diff generated using imagemagick)

DejaVu Sans,12,-1,5,50,0,0,0,0,0
failure