Because the Custom Event Field is event specific, not profile specific, it will only show through the initiative records. 
To view the content of the field, go to:

For a Form-
1. Communities > Forms>Your Form
2. Hover over the form name and select Bookkeeping
3. Click Manage Initiative Records
4. Search for your registrant or donor 
5. Click Edit
6. Click Edit Event Information tab
Your custom event field's content should show on this tab.

For an Event:
1. Communities > Forms>Your Event
2. Hover over the form name and select Bookkeeping
3. Click Manage Initiative Records
4. Search for your registrant
5. Click Edit
6. Click Edit Registration Information tab
Your custom event field's content should show on this tab.