Sometimes, if you update from an earlier version, old flexform settings could result in unexpected results.
Have a look at the famose extension “typo3_console” from helmut hummel to fix this problems by using the
cli command “vendor/bin/typo3cms cleanup:flexforms”.
Another way is to open the plugin over the backend, clear the content of the database field pi_flexform of the
corresponding tt_content record using e.g. PhpMyAdmin and press save inside the backend.