Odd mess with Junk and Spam folders

I am using a locally installed Outlook (2016) as my email client on a Windows 10 Laptop. I am accessing my mail provider using IMAP and I am witnessing an odd naming mess-up:

The folder where my Email provider moves Spam to (and which is called "Spam" when accessing emails via the provider's Web-interface) is called "Junk" in Outlook. That would be OK and I could live with that, although I am puzzled how and where that name mapping is done.

However, odd enough I do have an other folder called "Spam" (shown as top-level folder in Outlook) and that folder is shown in the the Web-interface as a nested folder under "Own folders/Spam". Annoyingly about 20% of all Spam message go to that second Spam folder. I was unable so far to figure out, why or by which mechanism emails end up in this folder. I would prefer if all Spam emails would end up in the same folder, because right now I always need to check TWO spam folders for possible false positives to not miss any legit emails.

That second spam folder, however, is not deletable (there is no Delete folder-option in its context menuin Outlook). It's also impossible to delete that folder in the Web-Interface. If I try so then it's gone at first, but if I refresh the email page then it reappears. 😞

Where is this name mapping done and how can I either name Outlook's Junk-folder "Spam" or get rid of that second Spam folder? Can one rectify this mess? And if so: how?