Flame Wall support gems are bugged.

Fortress specifically states that it reduces damage of HITS.

Flame Wall doesn't hit, because it says so in the description, and also it's impossible to use supports that are for "spells that hit" with it.

Yet, when I put Fortress into Flame Wall, it straight up cuts its Ignite damage in half.

Also for some reason it is possible to slot Considerate Casting into Flame Wall, which makes it deal more damage.

and in the description it says it does ignite, but i cannot use ignite support gems (ethernal flame, ignation, ..)

Also one of the recommended supports is Intense Agony, the one that increases damage of no-ailment damage over time. Ignite is elemental. Flame wall supposedly does ignite damage.