Oh so I guess that Flexify overwrites the domain then… :(( As I manage my products manually, the links in my case have not been overwritten and everything works for me.
Did you try to Play with Flexify settings on Shopify, perhaps there is a part in there that affects how often the data updates?
Alternatively I think you can try to experiment with the Data Sources on FB side - but I have not done that so I cannot advise with anything specific.