My configuration file always rollbacks to original one after I changed settings

The strange this is the config file is writable.

Even I change the file manually, it was still rollbacked after I do anything on backend configuration pages.

How do I solve the problem?

I've experienced similar issues. After upgrading to UddeIM 4.0 all my settings were lost (no big deal). However, it's been hit or miss whether the changed settings were actually saved.
These scenarios can happen:
Change setting - click save -> page refreshes -> setting is back unchanged -> refresh page -> setting is on NEW value
Change setting - click save -> page refreshes -> setting is back unchanged -> refresh page -> setting is on OLD value

Joomla 3.8.2
CB 2.1.3
UddeIM 4.0
PHP 7.0