Sometimes you change a setting in Magento admin and save it, only to find the old value still there. The cause may actually be outside Magento. For large configuration forms, max_input_vars is one possible cause.

By default, PHP accepts only a limited number of input variables. When that limit is exceeded, additional form fields are discarded before Magento receives them. This is common with extensive extensions and configuration screens.

You can inspect the current value using:

A typical value is:

You can increase the limit in the relevant php.ini :

Reload the PHP service afterwards. Note that the web server may use a different PHP configuration from the CLI. A correct CLI value does not guarantee that PHP-FPM uses the same one.

If Magento seems to save a configuration form but individual values vanish afterwards, inspect max_input_varsbefore searching for a bug in Magento itself.