Disfunctional save of portfolio permalinks

There seems to be an error with the portfolio permalinks when saving in the backend on page ‘Settings -> Permalinks’ (plugin version v2.21.2).

In the initial state in section ‘Portfolio permalinks‘ the option ‘Portfolio base’ may be selected. This also triggers that option ‘Custom base’ gets the value ‘/%portfolio_page_slug%/’ assigned. When saving this setting automatically the original option ‘Portfolio base’ will be unselected and instead option ‘Custom base’ will be selected with a slightly changed value ‘%portfolio_page_slug%/’ (no leading slash). But as far as I can say this setting works properly.

The problem occurs when saving the Permalink page again. Simply saving the page results in a wrong value for option ‘Custom base’. Instead of ‘%portfolio_page_slug%/’ now ‘%postname%/’ is set.

The wrong value than results in ‘404 Page not found’ errors.

Manually selecting the option ‘Portfolio base’ and saving will temporarily repair the wrong setting. But as soon as the permalink structure is saved and option ‘Portfolio base’ is not selected the 404 problem will reappear.