inches of water found in spare tire well

My 2017 Envision has also been collecting water in the spare tire well after rain fall. The Mechanic is saying its a clogged sunroof drain however only my trunk carpet, spare tire well and rear passenger side panel (the trim that follows the hatch) are showing signs of wetness. I was dismissed when I offered up the possibility that the weatherproof seal around the rear hatch may be the issue.