Comment 20 for bug 1644530

While I'd like to understand what systemd changed, I still think adding the PIDFile is correct as a fix for keepalived. Understanding systemd might just open up to fix more than just this service.

Never the less for type forking (that is the case here) PIDFile is recommended (https://www.freedesktop.org/software/systemd/man/systemd.service.html). Various cases are listed when the MainPID guessing might fail otherwise.
Also it is known proven to fix the issue, as well as being the upstream systemd file.

I'll next week schedule a fix and as well file one with Debian to not keep that Delta forever.