Resolved
Reported for: Toolset Forms 2.1
Resolved in: 2.4
When you disable Toolset Forms AJAX upload mechanisms for fields that support it (using the cred_file_upload_disable_progress_bar hook), then existing Featured Images are removed from Posts when edited with Forms where AJAX was disabled.
As a workaround, you can reupload the Featured Image before submitting the form. This will avoid the issue.
Other workaround would be not disabling AJAX form submission.