SQL Server 2016 still uses mixed_extent_allocation in TempDB
According to this post, IAM pages are still allocated from mixed extents, even in SQL Server 2016.
So this will be the mixed_extent_allocation I saw in my trace.

The blog post by Paul Randal provides some more internals explanation.
Thanks to Scott Hodgin for pointing me to the article.

I did soms more research about the content of de pages allocated in the first SGAM page and found out that SQL Server also uses mixed extents for system tables (object_id lower than 100).