That most likely means something didn't install correctly to begin with when you installed the packages that are listed in the 2.18 installation manual, or that something is broken with your Ubuntu installation. Most likely installing texlive-full fixed whatever went wrong when the other texlive packages were installed initially.
I have tested the packages carefully, and there is nothing more that is needed. Furthermore, this works in a docker build which only contains the basic Ubuntu 22.04 installation with these packages. That is essentially the same as running Ubuntu with webwork in a VM.
Note that it won't make a difference which of the serving options are used. The app itself runs the same in all cases, and how it runs pdflatex will be the same.
What about the perserve_temp_files documentation do you think needs to be updated? That setting preserves the temporary files created by webwork2. It would not have anything to do with any temporary files which may be created by the system elsewhere like the /tmp/texfonts directory and any files within.