We are operating a subdirectory-based multisite.
Every time we push from devkinsta to remote, and transfer from remote staging to Live, the ‘DOMAIN_CURRENT_SITE’ in wp-config.php becomes ‘undefined’.
define(‘DOMAIN_CURRENT_SITE’, ‘undefined’);
We want ‘DOMAIN_CURRENT_SITE’ to automatically replace itself with the appropriate value for Live, staging (with subdirectory changes each time), and devkinsta.
We are also operating a single site on another site, and it automatically replaces itself.
Hello Keisuke 
This seems to be a bug and out developers are checking it. I found the same when you first reported an issue with the Multisite and DevKinsta.
It’s under review, but there’s no ETA so I’m not sure when is this going to be fixed. However, we are aware of the issue and intend to fix it.
@VladimirM
Thank you for your message. I understand that the issue is a bug. Could you please let me know if there are any measures or solutions currently available to address this issue?
Hello @keisuke 
At this moment, the only workaround is to change the line manually in wp-config on staging site before pulling to DevKinsta.
Until the bug is resolved, there’s no other workaround, I’m afraid.