I think this chapter supports a theory I've had for a while
Toya is highly resistant to regular fire, if not immune. Toya's flames are much hotter than endeavor or shoto's and Toya has some resistance to his own flames, albeit his resistance is quite bad.
due to Toya being able to use flames of such great heat, normal flames don't bother him. Hence, Shoto was getting pushed back in the fight while Toya seemed relatively unscathed by Shoto's attacks.