After being shot, ghast fireballs explode and set fire onto blocks BEFORE contact with any surface. Fireballs can also be reflected by player before impact and still explode.
(Edit: am unsure if this is a Realms-only issue. May happen on singleplayer although I have not checked in singleplayer.)
(Edit, again: Have tested in singleplayer survival. This issue only occurs on realms.)