Yeah, that’s different to what I get when opening default normal maps in GIMP. It looks similar but mine almost look like a typical normal map but over saturated. I’m not entirely sure what format MSFS uses because I’ve never created a livery that I felt needed the normal map replaced.
In any case, to get a flat normal map this is what I would do. Create a basic texture with the RGB levels I previously mentioned or colour pick from my previous example, which is probably slightly off due to exporting as a jpg but would probably still be ok.
Then use MSFS to compile your project via the SDK with that flat texture in place as the normal map.
Something that is also important is having an appropriate json file alongside your texture but I imagine you are aware of this as you have a functioning albedo. Using the SDK to export the project will likely create them anyway.
Edit: Good luck. I’ll probably have a play about using the SDK livery creator tool. This is the only thing I had imagined I might need it for previously.