There’s a few things I notice:
A) The export format of 8-bit PNG could be incompatible with the import format or the material setup (less bits per pixel might mean it is brighter)
B) There’s no texcoord nodes connected to any of the texture samplers in the material.
C) In Texture Settings in Unreal (3rd screenshot?), try decreasing Brightness / Brightness Curve, and increasing any of the next 4 in the list (Vibrance thru Hue). Not by a lot, though. Vibrance is similar to Saturation, though it could brighten the result depending on how much Brightness is decreased. Hue is similar to Saturation in that it adds more of the current color to the output, I think.
In the foliage actor’s details panel of the grass, is it set to “Shadow Two-sided”?