It was a separate question.
Can't answer your question fully because I haven't reverse proxied plex, let alone two of them.
At first look, it seems that the /web location is shared between both plex servers and all requests are forwarded to one and not the other.
You might be better off proxying them via subdomains at the root so you don't have to worry about proxying additional subfolders like web.
Change your URL in letsencrypt settings to yoursubdomain.duckdns.org, set only_subdomains to false (should have been that way from the start, but I guess you followed an external guide rather than the description in the container settings, it is specifically spelled out there). Then you can set the subdomains to plex1,plex2
In the default site config, use the first server block for plex1.yoursubdomain.duckdns.org and the second one for plex2