Hi @maartenterhorst
There are no rules, only options and settings.
This is not an engraving question. This is an unexpected behaviour. The Dorico excerpt is needed to investigate why, in that specific occasion, your project is not acting as expected/desired. I tried to reconstruct your example, but was not able to replicate your issue (no unwanted clef appearing in my project): so it is necessary to see what is happening in your specific project, or we can only guess.
Here some information about this (specifically point 4):