At 18th level, you gain immunity to fire damage. In addition, any spell or effect you create ignores resistance to fire damage and treats immunity to fire damage as resistance to fire damage.
This ability allows you to treat immunity to fire damage as resistance to fire damage... and allows you to ignore resistance to fire damage.
Does the "ignore resistance" portion of this feature allow you to also ignore immunity that is turned into resistance by this feature?
Note: This is not official D&D 5e source material. Rather, it is something like Wizards of the Coast's "official homebrew"; it's un-playtested material by James Wyatt.