If the icons don't work when you use Leaflet through NPM, please report a bug at https://github.com/Leaflet/Leaflet/issues. This is certainly not an expected behavior. It might have regressed in recent versions.
Interestingly enough, the bug I was getting (can't seem to repro anymore) was already solved... by you! https://github.com/Leaflet/Leaflet/issues/766#issuecomment-7...
None of the fixes on that page worked, but using DivIcons did. If I can repro, I will definitely make an issue. Thanks!