Yes, indeed, I have the same problem.
Unfortunately, imho the docs about ingest.timestamp are pretty incomplete.
There's only one paragraph about it in
https://www.elastic.co/guide/en/elasticsearch/reference/current/accessing-data-in-pipelines.html
where's no information about format or options. As this "replaces" the automatic timestamp mechanic of elasticsearch, I think more information should be supplied.
Anyway, could you please so kind to share your script?
I'd like to keep the default timestamp format and don't change the mapping.
Best regards