This is an intended interaction. When an invisible enemy is hit with any source of damage it flickers revealing their location but not allowing them to be targeted. In this case it was probably the sunfire cape that triggered it.
Patch notes about this:
"Shimmer
Hitting stealthed units briefly reveals their location.
This pre-season, we wanted to create a way to deal with stealth that isn’t just “turn it off.” This new shimmer mechanic is a means of scouting for—and reacting to—stealthed enemies without completely exposing them to retaliation.
WHAT ARE YOU HIDING? Damaging an Invisible or Camouflaged enemy (ex. skillshots, area-of-effect abilities, etc) will cause the air around them to shimmer, momentarily revealing their location but not making them targetable or canceling the stealth effect
NO DISCOS Champions only shimmer up to once per second, even if they take additional damage during that time."