as a follow up for future readers I made a simple project using Mojave which I sent to Chris.
the project worked when compiled in 2018r4 on Chris system but not on 2019r1.
but on my system I have no issues with the same project when using Mojave and 2019r1 using my serial to USB converters and serial hardware connected to it.
For me the conclusion is that Mojave and 2019r1 are perfectly good for serial communications in all the testing I have done.
what ever the issue seen by Chris MAY be to do with the hardware he is communicating with, but standard FTDI USB to serial cables connected to hardware that DOES NOT use DTR/DTS seem to work as expected for me.