Thanks @v-yuazh-msft Alice for the reply!
That does not work, because fullname is not available in the dynamic content. That is because the fullname of the owner is "joined" with the "Expand Query" parameter of the "List Records" Action.
To illustrate why I think it is a bug, please try the following sample:
1.) Create an account in Dynamics CRM.
2.) Create a second account and make the first account the parent account
3.) Create a Flow and use Use List Records to get accounts with parent accounts and parent systemuser by using Expand Query parameter:

4.) Test your Flow

As you can clearly see: Flow correctly parses parentaccountid as an object, but does not do it to the owninguser. Owninguser becomes an escaped string instead of an object. My guess is, that this is because owningiser is splitted to owninguser, owningteam and owningorganization in the background.
Anyhow, thanks to your reply I was able to do it with a workaround by using Apply To Each Action. But before your Compose Action I had to do Get Record for all the linked Entities I want to get information from.
So instead of using "Expand Query" on the List Record, I get the related Entity afterwards in the Apply To Each.

So my problem is solved, thanks for the assistance.
But maybe you want to double check the List Control Action anyway, because its not really working as expected.
All the best,
Simon