Solution 1:
Check whether the user’s profile have read access to the related object field. Make sure they have Read as per Field Level Security.
Reference Article: https://help.salesforce.com/s/articleView?id=000317503&type=1
Solution 2:
1. Check the Lightning Record Page.
2. Check whether Related List – Single component is used instead of Related Lists component.
Note:
If Related List – Single Component is used, only the related object specified in that will be shown. If Related Lists component is used, then it will display all the Related Lists added to the page layout.
