DOIs not assigned when publishing an issue

Hello everyone!

We are using OJS 3.1.2.1 with the DOI and DataCite Export/Registration plugins properly configured (both tested). One of our user have recently published an issue of one of their journal, but DOIs haven’t been auto-assigned to the articles as we expected reading this:
https://docs.pkp.sfu.ca/doi-plugin/en/when-are-dois-assigned

Looking for some hints we found this thread (even if it refers to an earlier version):

where it is clearly stated that you must manually assign them. Is it possible that there was some error during the publication or is it correct that the DOI should be assigned manually to each article before the publication of the issue?

Thank you very much

Hi, any advice on this problem?
Reading this part of documentation regarding the quicksubmit plugin we understand that for articles uploaded by quicksubmit DOI must be assigned manually editing the uploaded purpose.
Are there other scenarios where doi have to be assigned manually, despite DOI plugin is set for assigning them with a defined pattern?
Best regards,
Stefano