AFAIK Leya should still remove the source collars even if you rescue Gareth first. Malady will remove them, if you don't do so otherwise before then.
Is Lohse in your party? She should have a unique dialogue option with Leya, similar to Beast's option with Duggan.
Please email supportdos2@larian.com with your latest save (or a Dropbox or Google drive link, etc), and one before rescuing Gareth, if available. If you are using any non-Larian mods, also include a list of those enabled and/or the modsettings.lsx file from your profile folder.
Each save is a folder in the '..\Documents\Larian Studios\Divinity Original Sin 2 Definitive Edition\PlayerProfiles\<ProfileName>\Savegames\Story' folder.
In Windows Explorer you can zip a file or folder by right clicking on it/them and selecting 'Send To | Compressed (zipped) Folder' (and similarly in OSX/Finder).
