After changing which Workspace is the default, via K2 Management, users still see the previous default when they load Workspace.
After defining a custom Workspace, setting it as the default, and marking the 'out of the box' K2 Workspace as "hidden", you may still see the OOB K2 Workspace as the default. Users who have never loaded Workspace before will see the correct default Workspace the first time they load it.
To resolve this, please reload your browser (CTRL-F5, for example). This is likely a caching issue.