Virtual AP Client Isolation Problem

I've noticed a strange issue with "Isolate Clients" on one of my vAPs.

Hardware is a Linksys WRT1900ACS running official 18.06.1, r7258-5eb055306f, all packages up-to-date.

When enabled, my Android clients no longer get Google GCM/FCM notifications (specifically from Signal) on time. Once I disable this option, the notifications are back to normal.

What routing changes are made when "Isolate Clients" is enabled that would cause this?