Hi @mikvaar ,

I know this is really confusing but this is expected behaviour. It's well described in documentation. So,as they stated below, run as can be even use in situations where  original user who created the pipeline has been deactivated - for example, if they left the company. 
So, basically SP configured with run as becomes new owner.

Configure Lakeflow Declarative Pipelines - Azure Databricks | Microsoft Learn

But maybe databricks should think about changing name of that feature. In my opinion it's doing much more than a name suggest and it's confusing, so I'm not surprised that you asked this question.