I got the same kind of issue.

You should see in the event log why or in some of workflow status why.

mine was the email sent was too big so it cause a reject and terminate the process.

the other was wrong email address.

You can fix this issue by adding catchup error

if you look in the message of why it cause issue by using workflow designer , in the common search you should find the instance that is terminated, and in the message you should see why it has an issue.