thanks for your feedback. I'll definitely try playing with the caching option in the plugin and see how that affects things. Regarding any other caching: everything is disabled. The site is still in development stage. No WordPress cache, no browser caching, no CDN, no server cache. Also been deleting browser cache like a maniac. So the plugin's cache could be the last suspect when it comes to that.
Enabling “Override content” makes the plugin behave in a predictable manner currently. If that is intended or not I don't now.
Multisite: I realize there are structural problems related to WP architecture. To me it feels that Multisite just got propped on as an extra feature once WordPress got bigger and nobody really bothered writing down what kind of user cases this could bring and then really think about why it should be really neccessary to keep the sites 100% separate. I find it simply mind boggling that there is no switch to access data cross site.
There's two main scenarios: you set up mulitisite for individual subsite users/admins like on wordpress.com and then security should be an interesting issue and keeping sites separate is a paramount goal. But then there's the other scenario: you use multisite for whatever reason and you are the only admin. Then there is absolutely no reason what so ever to deny cross site data access. But nobody seems to care and that's when things get messy.
My reason for going multisite with this install: inconsolable plugin conflicts – which seems to be the norm and not the exception with WordPress. And now I have to find a way back to cross connect data so it behaves more or less like a single site install again.
As much as I understand your explanation – the whole point why I went multisite was to avoid plugin conflicts. Which already is a serious flaw in WordPress core. So now you're solution requires me to activate plugins on subsites (that don't need the plugin) just so that your plugin knows what custom post types it needs to look for across the network. And all I can do is keep my fingers crossed that I can find a configuration of active plugins that doesn't break everything again and make me none the wiser as if I had stuck with single site install to begin with.
Don't know if there's a way around that. Like creating an index of all custom post types across the network that determines what post types Related Post Pro can find. Just a thought.
PS: I'm honestly sorry that I am not very relaxed when it comes to WordPress. Currently I have a really hard time telling what's worse: dealing with yet another WordPress issue or somebody ripping my nails out.