The following were observed after the join creation -
I was not able to open any of the profiles from the profile tab saying "ERROR - The resource you are trying to access is unreachable".
I viewed my custom resource table from client data and it was empty (even having records not linked to profile). Only those records were visible in which it had values in the attribute chosen for the link in both profile and custom resource.
Please advise or correct if this is acceptable or if any corrections needs to be done to correct both issues.
I believe you have created link from Profile to Custom resource, if that's the case then remove the link and recreate the same link from custom resource to profile. This should fix the profile access issue.