How to improve inverted text recognition?
First of all, you can improve the background using a custom Image Enhancement Profile.
You can convert the image to Black and White and adjust brightness and contrast. It's up to you to find the optimal parameters which will be acceptable to all samples.
If you need to improve the recognition results in FlexiLayout Studio, the custom Image Enhancement Profile could be done in Project Setup Station and then imported to FlexiLayout studio and used on image loading. For more information please check the following Guide: https://help.abbyy.com/en-us/flexicapture/12/flexilayout_studio/image_enhancement_profiles_
Also, you can try to use the Object Collection element in FlexiLayout Studio: https://help.abbyy.com/en-us/flexicapture/12/flexilayout_studio/object_collection. It has the Inverted Text option:
Finally, on the Document Definition level the recognition parameters for the field could be changed to invert the text: