(Could also be a 5.9 regression, didn't test it back then): 

When adding a new (existing ones work) pinned task and then manually sort it (drag it to a different place), it returns to the end of the list after a new window has been opened  (or that application closed and re-opened) 

To reproduce: 

create a new pinned application (e.g. open system settings and pin it, I pinned it for "all activities"), then move it to a different place  (e.g. to the left). Close and re-open the application, and it will move back to the end of the list (right, by default in a bottom panel)