Did you go back and double check each of the altars?
There was a known issue which could block the achievement after completing Alexandar's quest, but that should have been fixed a couple of updates ago.
If that doesn't help, please email your latest save, and (if available) one before praying at any of the altars (or a Dropbox or Google drive link, etc) to supportdos2@larian.com, with a description (or link to this discussion).
Each save is a folder in the '..\Documents\Larian Studios\Divinity Original Sin 2\PlayerProfiles\<ProfileName>\Savegames\Story' folder.
In Windows Explorer you can zip file(s) and folder(s) by right clicking on it/them and selecting 'Send To | Compressed (zipped) Folder'.
