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.
https://imgur.com/a/3swdIuf

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?