Personally, as a DM, I wouldn't treat the secondary 'attack' as a separate thing which prompts combat challenge - I would consider it ridiculous to do so; especially in the case you give (hits fighter, explosive burst damages other creatures nearby).
Just because mechanically it uses a second roll, it doesn't necessarily follow that it is a logically a separate attack for the purposes of combat challenge.
Now, if you (or your DM) decides that they want to follow any rules as literally as possible, then you might end up with a weird situation here, but that is down to them.
Regards,