Steps to reproduce the issue

I'm calling JUserHelper::setUserGroups from within a plugin.

Expected result

Users group membership changes and no other modifications are made to the user.

Actual result

Every value stored in the #__fields_values table for that user are deleted.

System information (as much as possible)

J3.8.2 PHP7.1

Additional comments

I had a feeling this had something to do with the fields system plugin, so I took a quick peek and noticed that the fields aren't updated if there is an input variable "task" with a value of "activate", "block" or "unblock". So as a test, I set "task" = "unblock" and the field values remain. Fortunately, this doesn't actually unblock a blocked user, so I can work around this issue.

Without looking too closely, I believe this is a problem in the fields plugin, onContentAfterSave event. I believe that 2 lines of code would fix this.

Wrapping $model->setFieldValue($field->id, $item->id, $value); with if(!is_null($value)) { } would probably do it.