Eevee Rendering Transparency Materials as White

I’m getting this odd effect when rendering out a foreground/background for my recent shot. I have a multi-part transparent object (the ship’s thrusters) which renders fine when the environment texture is in place… but when I render with film-transparency (so as to tweak the background and foreground separately in comp), I get only white flames on my thruster. This looks fine in my viewport render with eevee turned on and looking through camera. When I hit F12, it goes white.

This looks like an issue with how alpha is treated. When interpreting the rendered footage in your compositor, have you tried both “Straight” and “Premul” alpha types?