" clients get the public ip's from the vpn-provider."
The client is doing no such thing!!! pfsense gets an IP from your vpn..
So your vpn interface on pfsense if its sees traffic to 64603 it forwards it to your pflex server 192.168.1.8..
Here is the thing how does does rebooting your plex server have anything to do with that?? Nothing!! It has NOTHING to do with that.. You could reboot every device on your network and does not matter. You set pfsense to forward traffic it sees on this interfaces IP on this port to this 192.168.1.8.. Doesn't matter if plex server is running or not.. As long as pfsense can arp for the mac of 192.168.1.8 it would send the traffic there.
So what I would suggest is you figure out why when plex restarts your having whatever problem it is you think your having.. Again.. How are you trying to access plex server - from where?? Why don't you go to can you seeme.org and generate traffic to your IP that you have on your vpn interface.. To the port 64603.. Do you see this traffic at pfsense? Sniff - does pfsense send it on to 192.168.1.8??
You got failovers on your multiple wan options, you have an overlap in your manual outbound nat.. So which one is getting used the /24 or the /26 etc. etc.. Since that would be an overlap for your 1.8 address..