One theme runs through nearly all of this: Jira told you it worked.
A form smart value writes nothing and reports success. A default value of None vanishes and takes the field's behaviour with it. sendNotifications does not suppress the notification. A filter keeps returning rows long after the field it references stopped existing. An attachment migration ships zero bytes behind an HTTP 303 that everything in the chain treats as fine.
So a good half of these are really about verification — checking that what you configured is doing what you think, usually with a query you can run yourself. Where the answer comes back as a count of zero there is normally a note on proving the query could have returned something at all. A broken query and an empty result look identical from the outside, and only one of them means you are finished.
What to check when a Jira rule or API call finishes without an error and the change never actually happened.