Fire is denied from being placed by most normal means, but explosions which create fire (bed/respawn anchor/ghast) still create fire.

In 1.12.2, ghast fireballs still create fire, so this may be intended or just a really weird edge case.

Just gonna consider this as intended since technically fire has always been allowed in the Aether in some form and we cant completely cancel it out or else it'd have some issues with the Sun Spirit boss fight.