I have now tried this on two machines with the same result using the same project.
If export a file using the deliver page - I chose to just make an audio file and ticked the subtitle creation - Resolve makes an audio file and subtitle file.
If I use export subtitle from the file menu or timeline after exporting the file from the deliver page, it works. Do you need to have exported a file from the deliver page for the other options to work?
My two computers:
Computer 1 - Windows 10, 32GB RAM, nVidva 6GB 1060, Intel Kabylake i7.
Computer 2 - laptop - i7-6700HQ, 16GB RAM, 2GB nVidia 980M.
The log from computer 1:
http://www.dvctraining.co.uk/downloads/DaVinci-Resolve-logs-20180910-121309.zip