Were many of your FB conversions click-through conversions before the change? If not, it might be the iOS privacy settings blocking your pixel tracking for view-through conversions.
Are you targeting iOS users? A lot of the conversion data for those users will be lost if they decline to share data. Also, I read here that in some countries the sharing option is automatically set to not share. If you’re in one of those countries I would think it would affect you a lot faster than most.