There is already a comment about this in rc.openvpn. I can confirm that often when doing:

nothing happens. I'm not sure why, but the openvpn process seems to change pid at some point, and then whatever is contained in /run/openvpn/*.pid is out of date. I agree with the suggestion from rc.openvpn that 'killall openvpn' should be used instead. Could this plea make it to those in high up places please :-)

Which version of Slackware are you running?

Is this on a server or client?

I'm not sure if this is what you're asking, but the rc.openvpn script in Slackware64-15.0 is designed to use "killall openvpn" in the right conditions.

Perhaps I'm a relative newcomer, but we've been using OpenVPN pretty much 24x7 here since around March 2020 and haven't seen the behaviour you describe. Our main OpenVPN server has been up for 524 days, with 6 users on it for 8 hours a day and the PIDs of the openvpn processes match the contents of /run/openvpn/*.pid. In all honesty, I can't figure out how you'd get into a situation where they don't...

What exactly are you seeing? Are there any errors in your logs which might help?