well, who says they can't just see their own reflection?

the only reason Simba saw Mufasa the second time is cause Rafiki touched the water and made it all magical lol. and the only reason Kovu saw Scar was cause he had that scar then and everyone kept trying to compare him to Scar. besides, Scar isn't related to Kovu at all.
as to Kiara, that was trying to get her to understand the we are one thing with the Outlanders and Pridelands, as well as her and Kovu were meant to be together. if Mufasa could knock that gourd off Rafiki's stick and crack it to give him the message for his plan to have Kiara and Kovu together, then who says it wasn't him making half of Kiara's reflection disappear hmm?