Normal map DirectX... or is it?

Summary

sketchfab expects directX normal maps according to the information on the platform. exept…I’ve tried several software with normal maps, like metashape and Cinema4D that export openGL normal maps. In sketchfab these normal maps don’t have to flip the normals. So my question is: what is happening? Is sketchfab converting automaticly. I’ve tried several normal maps.
Rene

What type of bug are you experiencing?

Other

Steps to Reproduce

upload a openGL normal map to a mesh. check if the green channel must be flipped. It should be if sketchfab is expecting DirectX. in my case it doesnt need to be flipped.

Expected Result

normal maps should be directX according to details provided by fab.

Observed Result

sketchfab expects directX normal maps according to the information on the platform. except…I’ve tried several software with normal maps, like metashape and Cinema4D that export openGL normal maps. In sketchfab these normal maps don’t have to flip the normals although they are openGL. So my question is: what is happening? Is sketchfab converting automatically. I’ve tried several normal maps.
Rene

Platform

windows10 firefox

Operating System

desktop windows10