After upgrading to CMS.12.18.0, when navigating to the Setting Tab for a job, the Start Date (UTC) fields are disabled https://www.screencast.com/t/HfjUODbhB5.
Using the dropdown selector, makes no difference because when you hit the Save Scheduled Job button, the isEnabled property is sent as false to the server https://www.screencast.com/t/CYGWzNR0a4.
I can schedule with Alloy site. I'd suggest to contact developer support service, it might be something specific to your site/scheduled job
Thanks for looking into this so quickly, you were right, issue was project specific