It looks like there's a conflict with the navigation of nested tabsets.
echo JHtml::_('bootstrap.startTabSet', 'outerTab', array('active' => 'x1'));
echo JHtml::_('bootstrap.addTab', 'outerTab', 'x1', 'x1');
echo 'Outer Tab 1';
echo JHtml::_('bootstrap.endTab');
echo JHtml::_('bootstrap.addTab', 'outerTab', 'x2', 'x2');
echo 'Outer Tab 2';
// Nested tabset
echo JHtml::_('bootstrap.startTabSet', 'innerTab', array('active' => 'y1'));
echo JHtml::_('bootstrap.addTab', 'innerTab', 'y1', 'y1');
echo 'Inner Tab 1';
echo JHtml::_('bootstrap.endTab');
echo JHtml::_('bootstrap.addTab', 'innerTab', 'y2', 'y2');
echo 'Inner Tab 2';
echo JHtml::_('bootstrap.endTab');
echo JHtml::_('bootstrap.endTabSet');
// End of nested tabset
echo JHtml::_('bootstrap.endTab');
echo JHtml::_('bootstrap.endTabSet');
I did a LOT of work on nested tab state in Joomla 3.x, however Im guessing that this is something brand new in Joomla 4 thats not using what I did...