Express Forms Not Sending Notification Emails
I'm guessing if you update to the latest 3.x release of phpdotenv, the issue will resolve itself.

3.x added better support for environment variables running in multi-threaded environments.  You can read some background info here: 
 https://github.com/craftcms/cms/issues/3631

Note that you'll likely need to update how phpdotenv is loaded in your public index.php file. You can read the upgrade guide here: https://github.com/vlucas/phpdotenv/blob/master/UPGRADING.md