I haven’t had that happen in CC 2018 but media offline usually means that, for some reason, Premiere can’t read the file. What format is the file? Are you rendering using previews? Have you checked that the file and/or folder has proper read/write permissions?
That said:
[david payne] “Watching every second of everything I produce is indeed an option, but it really shouldn’t be necessary. Sadly i have now lost all trust in premiere. “
Shit can and will happen. In every system with pretty much any program. That’s why I will check or spot check everything I send out to clients. If you have longer videos you could run them through MediaArea QCTools or something like that.