I have got mine to work by adding both domain names in the app domains field without the www prefix. My website was originally setup with say, aaa.com.au. Subsequently we parked domain bbb.com.au on this website and this is the website we are referencing. Easier to park the domain than change the whole website setup with the hosting service.
I have a FB page for bbb and set the FB app to point to https://www.bbb.com.au in the site url. However it wouldn’t work with just bbb.com.au in the app domain field but when I added aaa.com.au to this field (i.e. it now reads bbb.com.au aaa.com.au) it worked.
Not sure if your situation is similar but it may help.