saving inputfield config values via api fails

[jploch]

Hey!
Today I discovered a strange behaviour with a fieldtype module Iam currently developing.
I have a function that updates the config of the corresponding inputfield module, this functon is called via ajax to update one of the config options (a textarea) with some data. The strange thing is that sometimes it works and then sometimes it saves the new value but deletes all the other config options. Any Ideas why this could be happening? Here is the code that saves the inputfield config, this code is inside a process module and gets called via ajax: 

$settingsName = $this->sanitizer->text( $_POST['settingsName'] );
$settingsValue = $_POST['settingsValue'];

        if ( !empty( $settingsName ) && !empty( $settingsValue ) ) {

            $f = $this->fields->get( "pgrid" );
            $f->set( $settingsName, $settingsValue );
            $f->save();
            return;
        }

(testet on lates PW master Version 3.0.184)