Steps to reproduce the issue

create a content plugin that triggers onContentAfterSave
My plugin works fine in backend (any Joomla or PHP version)
In the frontend it works fine with PHP 7.0.x. It breaks and shows error message with PHP 7.1 and above

Expected result

No error

Actual result

Too few arguments to function PlgSystemFields::onContentAfterSave(), 2 passed in /home/.../public_html/libraries/joomla/event/event.php on line 70 and at least 3 expected

Call stack

Function Location

1 () JROOT/plugins/system/fields/fields.php:43
2 PlgSystemFields->onContentAfterSave() JROOT/libraries/joomla/event/event.php:70
3 JEvent->update() JROOT/libraries/joomla/event/dispatcher.php:160
4 JEventDispatcher->trigger() JROOT/components/com_gsimporter/models/logform.php:350

System information (as much as possible)

Joomla 3.7, 3.8 and 3.9 - error irrespective to Joomla version
PHP 7.0.x no error
PHP 7.1 and above with error

Additional comments

original Joomla code is:
JROOT/libraries/joomla/event/event.php:70 => return call_user_func_array(array($this, $event), $args);

I suggest to add an argument to eliminate error - this solves the issue:
return call_user_func_array(array($this, $event), $args, 0);