Hi Micha!
DL;DR: Yes, exactly.
This has been a problem “forever”. The rulespec is not only used to render the values configured by the user, but also to render the “effective parameters for this service” (which may include discovered parameters) and the discovered parameters themselves (during discovery).
If some of the discovered parameters are not known to the rulespec, the error you described will occur.
Often plugin developers don’t enable the “Parameters for this service”-view, and so this problem goes by unnoticed.
In the past there where only two solutions to this problem: Either just add the key to “ignored_keys” or implement it as “regular” part of the rulespec. In the former case you don’t get nice rendering, in the latter you make the value user configurable, which might not make sense (but keep the enforced service rule in mind!).
This is one of the intended use-cases of the render_only attribute.
Thanks for your feedback! (We’re also looking into some other issues you’ve raised.)