Hi,

It looks like some kind of effect in your Unreal project or level, that makes the billboard's edges outlined.

You could try turning off Lit, Render To Depth, Alpha Correction on the Billboards. Also, you could try turning off the Light Warp if you turned it on:

If these don't help, you could try turning off Allow Virtuals. If that helps, that means the problem is caused by an effect in your Unreal project or level. If you want to use Allow Virtuals in this case, then you could disable things in your level to find the source of the effect. In Live Sync mode you can do that fast without the need to recook your project.

Warmest regards,