I’ve always pronounced it "-tile", not "-teel" & both Wiktionary (https://en.wiktionary.org/wiki/mercantile https://en.wiktionary.org/wiki/mercantile ) & the OE

I’ve always pronounced it "-tile", not "-teel" & both Wiktionary (https://en.wiktionary.org/wiki/mercantile https://en.wiktionary.org/wiki/mercantile ) & the OED ( https://www.lexico.com/definition/mercantile https://www.lexico.com/definition/mercantile ) agree that it’s pronounced "-tile" rather than "-teel", so I think your initial belief was the correct one! Wiktionary suggests that -teel is an alternate pronunciation in the US, but that -tile is the preferred one even there.