In older releases, this used to work only for software bridges. In your router, the switch is handled by a DSA driver and just represented as a bridge, the communication between your hosts is handled by the switch hardware. The isolate option is probably not passed down to the hardware (I haven't verified this, but other forum posts suggest so).
AFAICT, you will have to set up individual VLANs or interfaces for every port you would like to isolate.