(I use this feature every day, multiple times a day, to export vector line art to PDF files - and have NEVER had this issue until now).
All vector lines are being transformed into bitmap rendered line segments (they are several pixels wide and have gradations in shading within the width and length of the lines). No options available under 'Publish to PDF/Settings' dialog box to remedy this issue. Tried every option under the 'PDF Preset' dropdown (Archiving, prepress, document distribution, etc), but that did not fix the issue. The dialog box also shows "1 Issue detected in document', this issue is 'there are fonts below 18pt in your document'. I don't know why that would affect the vector lines, but it's worth noting as I did not have this issue before the update.
Work-around: The only work-around I have found is to use the 'Print' command, then change printer to 'Adobe PDF'. This results in vector lines remaining in the final pdf, not being converted into bitmap. This is, however, not the same as Publish to PDF, and takes significantly longer to 'Print' the file (ie, it's not an ideal workaround).
My last test using merge modes definitely shows that merge modes are an issue. Now whether that's a limitation of the PDF format or a limitation of the Draw publish to PDF I cannot say.
There is an issue with the size of the object also, even with normal merge mode, a .001" weight outline converted to an object works but when a lens is applied the objects develops breaks even in normal merge, it remains a vector.
I do believe thete is a minimum limitation in PDF if I recall.