I spent last 2 full days trying to figure that out, and no luck.
I have low poly and high poly version of my model. Low poly is triangulated beacuse i want to texture it in Substance Painter. I baked normal map in Blender and it is working perfectly fine inside both eevee and cycles (1st picture)
The problem occurs when importing my low poly to substance painter or sketchfab, or I imagine to any other software. Importing model in those programs results in (if i understand the issue correctly) loosing custom split normal data or autosmooth and the normal map looks off (2nd picture).
Tried every solution i could find, the most obvious being:
-Trying countless times diffrent export options, FBX, OBJ, Smooth Normals Only \ Face. Smoothing Face only (option while exporting) seems to work best but still it has issues.
-Trying edge split instead of custom normal data, semi worked but normal map had awfull black seams (even in Blender) and i couldnt get rid of them (cage didn't help)
-Baking in Substance instead of Blender
-I'm using OpenGL format on import in substance(so issue is not coming from choosing DirectX)
The reason i suspect "custom normal data" to be an issue is beacuse the bad looking normal map can be recreated in blender when deleting Custom Split Normals Data and turning off autosmooth, or just turning off autosmooth (however i checked and substance painter seems to acknowledge autsmooth so its most likely just Custom Split Normals Data issue). But maybe i'm wrong and the problem lies somewhere else? (3rd picture)
Possibly important note: somewhere in the process of modeling i applied Weighted Normals modifier.
Sending normal map.png and blend file with Low poly and high poly(hidden in scene). Triangulate modifier is not applied just in case someone would want to check if thats the problem or something similar (but of course I was searching for solutions with triangulation both applied and not applied (other programs do it either way).
https://files.fm/u/wbthvstaq (link to .zip with Normal map and Blender file)
I'm completly out of ideas how to fix it. Could you guys help me?