What the heck is going on? Milestone mayhem!

[Pet]

(see attached image) I can't figure out why the milestone in green, which clearly belongs to another workspace (Lill), is appearing in this (Brid) workspace?

When I go to the Lill workspace, Tasks, choose that milestone, edit, the workspace it is assigned to is Lill.

Can someone explain why it is appearing under Lill?

Everytime I think I have a grasp of how OG works, I get another curveball..!

cheers

[Pet]

Update to this: I checked it again today (one week later), and that milestone no longer appears under the incorrect workspace.  No idea why.