One solution is to change your font to bold and then when you export it will be non-bold! You can select all the relevant cells and the press COMMAND-b to do this. The only problem is that this problem does not occur for Greek font so you need to only change the non-Greek font to bold. I admit this is not very satisfactory as it is quite time-consuming and will need to be undone once the bug is fixed.
Here is an example notebook where this trick worked ( Ithink the FontColor commands should be removed as their colors have not copied over correctly):
Notebook[{Cell[CellGroupData[{Cell[TextData[StyleBox[test ,FontColor->]],Title,CellGroupingRules->{GroupTogetherGrouping,10000.},CellChangeTimes->{{3.63426*10^9,3.63426*10^9},3.63426*10^9,{3.64279*10^9,3.64279*10^9},{3.69655*10^9,3.69655*10^9},{3.72834*10^9,3.72834*10^9},{3.75287*10^9,3.75287*10^9}},FontFamily->Helvetica Neue],Cell[TextData[StyleBox[This is a test.,FontFamily->Helvetica Neue,FontSize->36]],Item2,CellGroupingRules->{GroupTogetherGrouping,10000.},CellChangeTimes->{{3.63614*10^9,3.63614*10^9},{3.66655*10^9,3.66655*10^9},{3.66716*10^9,3.66716*10^9},3.68332*10^9,{3.72834*10^9,3.72834*10^9},{3.72834*10^9,3.72834*10^9},{3.72835*10^9,3.72835*10^9},{3.72835*10^9,3.72835*10^9},{3.75287*10^9,3.75287*10^9}}]},Open]],Cell[BoxData[RowBox[{NotebookGet,[,RowBox[{EvaluationNotebook,[,]}],]}]],Input,CellChangeTimes->{{3.75287*10^9,3.75287*10^9}}]},NotebookAutoSave->True,WindowToolbars->EditBar,WindowSize->{993,929},WindowMargins->{{128,Automatic},{Automatic,120}},PrintingCopies->1,PrintingPageRange->{1,Automatic},PrintingOptions->{Magnification->0.55,PaperOrientation->Landscape,PaperSize->{1530.91,1081.82}},Magnification:>0.75 Inherited,FrontEndVersion->11.1 for Mac OS X x86 (32-bit, 64-bit Kernel) (April 27, 2017),StyleDefinitions->Notebook[{Cell[StyleData[StyleDefinitions->Default.nb]],Cell[StyleData[Text],FontFamily->Helvetica Neue,FontSize->36,FontWeight->Regular,FontSlant->Plain,PrivateFontOptions->{FontPostScriptName->Automatic}],Cell[StyleData[DisplayFormulaNumbered],TextAlignment->Center,FontFamily->Helvetica Neue,FontSize->36,FontWeight->Regular,FontSlant->Plain,PrivateFontOptions->{FontPostScriptName->Automatic}],Cell[StyleData[Subsection],FontFamily->Helvetica Neue,FontSize->40,FontWeight->Regular,FontSlant->Plain,PrivateFontOptions->{FontPostScriptName->Automatic}],Cell[StyleData[Section],FontFamily->Gill Sans,FontSize->48,FontWeight->Regular,FontSlant->Plain,PrivateFontOptions->{FontPostScriptName->Automatic}],Cell[StyleData[DisplayFormula],TextAlignment->Center,FontFamily->Helvetica Neue,FontSize->36,FontWeight->Regular,FontSlant->Plain,PrivateFontOptions->{FontPostScriptName->Automatic}],Cell[StyleData[Subsubsection],FontFamily->Helvetica Neue,FontSize->36,FontWeight->Regular,FontSlant->Italic,PrivateFontOptions->{FontPostScriptName->Automatic},FontColor->],Cell[StyleData[ItemNumbered],FontFamily->Helvetica Neue,FontSize->36,FontWeight->Regular,PrivateFontOptions->{FontPostScriptName->Automatic},FontColor->],Cell[StyleData[Subsubsection],FontFamily->Gill Sans,FontSize->36,FontWeight->Regular,FontSlant->Plain,PrivateFontOptions->{FontPostScriptName->Automatic}]},Visible->False,FrontEndVersion->11.1 for Mac OS X x86 (32-bit, 64-bit Kernel) (April 27, 2017),StyleDefinitions->PrivateStylesheetFormatting.nb]]