One disturbing thing about Facebook promoted / boosted posts is that you can't have too much text on the associated image. I get the idea behind that (to avoid promotional banners) but that also prevents me from using useful annotated screenshots too!
But there's nothing really you can do about that: The rule is the rule. What we can do is to avoid rejections by providing an image that complies with Facebook editorial terms of service.
This tool will help you with that: Facebook Image Text Detection Tool
Before promoting any post at Facebook, run your featured images using that tool to make sure you are safe!