A user needs to share an important segment with a group of users in a Product Profile Group. Why can't a member of his Product Profile Group view the segment?

The reason why a member of his Product Profile Group cannot view the segment is that the user forgot to select “Make this segment public”. When a segment is created or edited, there is an option to make it public or private under the Share With section. If the segment is private, only the creator can use it. If the segment is public, it can be shared with all users, product profile groups, or individual users who have access to the same report suite. The user did not need to curate or duplicate the segment.

https://experienceleague.adobe.com/docs/analytics/components/segments/segmentsharing.html?lang=en

https://experienceleague.adobe.com/docs/analytics/components/segments/segmentsharing.html?lang=en#sharing-a-segment