How can i set a user as Purchase Order manager or viewer?

Summary:

I cannot see or manage purchase order if the user has a custom role

Content (please ensure you mask any confidential information):

I set some users as view-only Purchase order and some as manage purchase order, they used to work since yesterday when, if searching for a purchase order, i get no results.

Am I missing some important role or privilege?

I also noticed it may be related to the business unit even though i gave to all the users the related data access,

what am i missing?

thank you!

Giorgia