Hi Alberto,
We appreciate that you posted back and shared the detailed information with us.And we read your description, it seems that some views and some users are affected by this issue. At the same time, if you removed Group by in some views, they will work, if you don't remove the Group by, nothing will appear.May I know if my understanding is right? If so, for the situation you encountered, we do understand the inconvenience caused and apologize for it. And we have also tested it on our side, we created different views and used Group By and Filter function in SharePoint list, and it works fine, we cannot reproduce the same situation.Given this situation, we're sorry the inconvenience caused by it.Since you mentioned it worked before and multiple users have the same problem, and we cannot reproduce the same situation, this issue may be related to your SharePoint and it may need to be checked from background. The best way to troubleshoot the issue is to report the issue to the related Team. We’d suggest you contact your office 365 administrator to go to Office 365 Admin Center-> Support-> New Service Request, the support engineers there have the correct escalation channel and this is the most efficient way to report such issue. You can refer to this article to check the way how to raise a ticket. Ways to contact support for business products - Admin help.
Your understanding and patience will be highly appreciated. I hope that you are keeping safe and well!
Best Regards,
Sukie