/who NEVER detected layers. Stop spewing this nonsense.
Source: Layer switching is the problem, not layering itself - #20 by Pazorax-1880
Pazorax
WoW Developer
Another popular theory is that you can determine your current layer by doing a /who in a capital city, and comparing the results against the /who your friend does. That doesn’t work because /who returns results from the entire realm, not just from your layer, and if the result set is too large it truncates the results before sorting them. This means every player gets different results, but those differences in result set have nothing to do with which layer you’re on.
The /who addon API was deliberately broken to stop gold sellers from automating chat channel invites to advertise their websites. It’s a sh*tty fix because it breaks some good addons like CensusPlus, but at the moment it’s for the best.