Microsoft project management software used to plan, manage, and communicate a project schedule and other information among workgroup members, project managers, and other stakeholders.
Hi,
Thank you for spending time doing test and sharing the detailed steps and confirming the scenario with us.
We have finished tests of changing Date format in Project Online, we find we can reproduce the same situation as you describe. Although we have set UK date format, but the dates in projects still display as US date format. We have waited 24 hours but date format still doesn't change.
We are very sorry that this behavior causes much inconvenience and apologize for it. In this situation, we sincerely recommend you let your Microsoft 365 admin go to create a support ticket in Microsoft 365 Admin Center > Support > New service request. The support team there has the correct escalation channel for this case. They have higher permission to check this situation from background and do investigation further. This is also the most efficient way in handling this kind of case for you.
For reference: How do I find my Microsoft 365 admin Get support | Microsoft Docs
We appreciate your patience and understanding. Hope you are keeping safe and well.
Best regards,
Tina