Bug Face missing from model after importing in .fbx format

Hi everyone.
So I modelled a gun model for my game in blender, and after importing it in Unity one of the faces simply disappeared as you can see in the link below :

https://ibb.co/rwPDY8r

I tried flipping the normal on that face in blender , and half a dozen of other solution for that problem I found online, but nothing works.

it's weird because it does appear normal in Blender :

https://ibb.co/BVdMGqg

One thing I noticed is that when I open the .fbx file with windows 3D viewer, this anomaly is visible on the hidden face :

https://ibb.co/1ZLxf0D

It is the only face on the model that has that black anomaly, all the others are normal.

I am on the verge of just deleting it and starting over., so if anyone has a solution I would be very grateful since I already spent a few hours making the gun and a couple more trying to fix this, I would hate to have to redo everything again (Especially since i'm pretty proud of that gun).

Thanks in advance.