You'll need to post a screenshot to clarify the issue.
You should enable debugging on the microflow that retrieves the parent's e-mail address. See whether the parent entity is present, and if the e-mail address exists.
If it does exist but does not show up, you might have an issue with the access rules. But also check the mendix tutorial videos. Your remark that "The selected record has a parent email column which is association with another entity" gives the impression that something might be designed under a wrong assumption.