Out of curiosity, does the Post Preview page redirect as well?
If it don’t redirect, then it sounds like a cache problem.
If it does redirect, there are more things to try.
NB: First back-up your site (always a good idea before debugging anything.) As you may not know what the developer edited, back up all files and the database as well.
If you have any caching / cache plugins, clear all cache files and then deactivate cache plugins.
Check status of the redirect.
If it’s still there, then try re-install WordPress. Login and go to Dashboard –> Updates. Click the Re-install Now button.
Check status of the redirect.
If still there, then you might have to consider deactivating plugins (one at a time) and check which could be causing the problem. I would normally start with any SEO plugins and Cache plugins.
If still nothing, then deactivate the rest, one by one, and check the redirect after each.
If no change, then it seems to be an issue with the Theme.
You can try change the theme to test. ** Please note, sometimes changing the theme can reorganize the widgets and cause some widgets not to show. So if you have a lot of special widget configuration, then you might want to hold off. Or if you are confident you can restore the site from back up, then you can give it a shot.