Well, something even we are seeing since the change in FB ecom system. We went from $1.90 a conversion to $26 a conversion overnight. Our ads have literally stopped working and even our FB ad support guy is scratching his head not knowing what has happened. I would understand a gradual increase in CP conversion, but not an almost $25 increase overnight and it has only gotten worse over the past 3 weeks. We've had our store for over 3 years now and the highest CP conversion we've ever seen was $3.6. Something very odd is going on.
You need to fully investigate the tracking setup. Did the sales within Shopify and Google Analytics reflect this drop?
If it didn't, it's good to review your FB pixel. Shopify's guide is a good place to start. There are multiple options that could have changed which reduced the conversion data coming in.