Additive alpha not as expected

I’m porting my game from OpenGL to Unreal. I use additive alpha to create effects in the game. However, Unreal doesn’t seem to add aplha in the same way. Here is an example:

You can see that in OpenGL a lot of overdraw cases the colour to keep adding towards it maximum.

In Unreal it seems to be limited in someway. Take the 3rd row for example, this is grey added to itself a number of times. In OpenGL this creates white, which is what you’d expect. But in Unreal it never gets past grey. It’s defeinlty lighter than the original draw colour, but something seems to be limiting it.

Even a single draw seems to be different. In OpenGL you can see the roll off towards the sides, but in Unreal the sprite is pretty much a contant colour until right near the edge.

Any ideas?

Btw, I use the following blend in OpenGL:

glBlendFunc(GL_SRC_ALPHA, GL_ONE);