Go to View Issues, select project P2 (in the Project selection box, top right)

Click on a bug that has the tag, click on the tag to get to Tag Details.
Problem 1: P2 is still selected, but the link at top right says "Attached Issues (3)" -- the count there seems to always include all bugs with the tag, not just in the selected project.

Click on this "Attached Issues (3)" link. Expecting to see either all 3 bugs, or only 1 bug in P2.
Problem 2: the 2 bugs in P1 are displayed! The Project box still shows P2.

Now I want to go back to View Issues. Clicking "View Issues" link.
Problem 3: The list of issues is unchanged. It seems that viewing that "Attached Issues" list has permanently modified the filter. That's not what I expect -- e.g. from "My View", clicking "Reported by Me", displays a list with a filter where Reporter is set to me, but clicking "View Issues" from there resets the filter back to what it was.

Trying to change the filter manually to include all issues in P2, which is still selected in the Project box. Clearing the Tags field, "Apply Filter".
Problem 4: The list includes issues from ALL projects, even though P2 is still selected.

At this point, clicking either the "Switch" button next to Project box, or the Reset Filter button, brings the list back to consistent state, showing only P2 issues.