That is completely normal. These are most likely all servers operated by Plex which simply test if they can reach your server from the outside.
Of all those servers, later one will be chosen (the one with the best network roundtrip time and therefore geographically closest to you) to be your assigned ‘pubsub’ server which will do all the authentication for you.
Also, as soon as you have shared your server with other users (or you yourself have plex clients running outside your home network), there will be regularly accesses from these clients to your server.
Accessibility from the internet is what ‘remote access’ is about.
