Featured images not showing on Facebook IOS app

I’m not sure if this is a Yoast issue, or related to theme or Facebook, but I noticed that several featured images on my articles don’t show when sharing the link on Facebook. This seems to be sporadic (images are same size, format but some are showing and some not) and is not happening on desktop, only on the Facebook app (I’ve only been able to try iPhone version). The Facebook debugger is pulling them through fine (although not every time – this, too, seems sporadic). Any help/advice welcome!

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