I just realised an email created by a person with higher level of org unit hierarchy can not be edited by person who has lower level even if the access authorization was updated to lower level. Whenever this email is edited e.g. test send, an error saying you dont have the write access to this delivery will appear.
So I'd recommend checking with another user who had lower access or logging in with a different browser in private/incognito mode. If either of these options allows you to view the email then it's just a cache issue meaning you'll need to clear the user's browser.
If the email is still restricted then I'd submit the issue to Campaign Support, since I'd expect that updating the email or user permissions should allow the email to be visible. This could mean a potential bug.