Just tried testing in a new project and it works fine for me. RBGA8 render target looks fine in editor and the resulting PNG matches when I export it via Blueprints and via right-click menu.
What happens when you export it from the Content Browser? Just want to remove gameplay and Blueprints from the equation.
What size is the resulting black PNG, and have you tried to evaluate it in something like Photoshop? A few kB would mean pure black with no data. Many kB might mean alpha channel is just 0.