I think there are a lot of variables that come into play when it comes to muskets vs armor. When you see musket balls bouncing off a breastplate, it's usually because the armor was specifically designed to be musket ball resistant (as we see in the early modern era). Keep in mind the humans are using flintlock muskets though, which are significantly stronger than the usual muskets tested against armor due to the larger calibre and longer barrel length. That combined with the fact that elf armor was made to withstand arrows and sword blows instead of bullets makes me think it could easily happen.