That's not the rule, patents have to be reproducible by a man of the trade. Not disclosing crucial steps in the claims, even if overlooked by an examiner, is also a handicap: someone can circumvent your patent if the actual substance is missing.

Have a look at the original Gore membrane patent: https://patents.google.com/patent/US4194041A/en

It's very clear, detailed and reproducible.

And since we are on HN, here a bit on the software side: IBM original arithmetic coding patent.

https://patents.google.com/patent/US4905297A/en

Perhaps it something that should not have been patented, but it's fully disclosed and reproducible.