even, if you update the page, the number is not updated.
As far as I've noticed, this behavior is caused by the DokuWiki cache. (Presumably DokuWiki assumes that the page was not changed and a change in the metadata is not registered because not checked.)
Maybe you can improve it by using the ~~NOCACHE~~ option.
In any case I consider this to be a bug or at least something which could be improved. What do you think about that?
I think that's a matter of specification. The auto-increment function, as implemented, is suitable for single using, not for multi-user with concurrent use.
I think it's up to you to remedy this. You have the need and the financial benefit. You or your company should program a better solution or pay someone to do it:
https://www.dokuwiki.org/faq:support#professional_support_and_services
I think it's not part of the core competency of a wiki that you can use it to write invoices in group work. But even if it were, DokuWiki is what it is and everyone who wants more needs to take care of it.
But who knows, maybe you can find someone to help you selflessly ...