nano ProjectZomboid64.json  In /var/log I haven't found any log related to PZ nor by running journalctl for the respective user.  

Also is are there any reports in /var/log that might give some clues?

How much RAM are you allocating to the server?

I just said that we had more mods in .18 and it worked fine.  We are using the vanilla 70 limit in both cases.  

Additional info: If one person is driving, the cones and blackwall are fairly uncommon but as soon as two or more start driving (near or far) the issue becomes as Beornvig describes. Also, speed does not seem to matter as each player could be going 30 or 100 but the issue manifests regardless.