Reproducible finding and workaround
I discovered that the presence of a subfolder inside the pinned folder determines whether the problem occurs.
- With no subfolders inside, opening it from the taskbar's Pinned list leaves the navigation pane at Home.
- Creating an empty subfolder inside the folder makes the pinned entry work correctly.
- Renaming that subfolder has no effect; the pinned entry continues to work.
- Removing the subfolder causes the original problem to return.
- Re-creating a subfolder restores the correct behavior.
The folder contains thousands of files, but no subfolders in its normal state.
This behavior is repeatable. The subfolder workaround is effective, but an empty folder should not be necessary to make the navigation pane behave correctly.