+static int rtl8366rb_port_set_isolation(struct realtek_priv *priv, int port,
+                                       u32 mask)
+{
+       /* Bit 0 enables isolation so set this if we enable isolation
+        * any of the ports an clear it if we disable on all of them.
+        */
+       if (mask)
+               mask = RTL8366RB_PORT_ISO_PORTS(mask) | RTL8366RB_PORT_ISO_EN;
+
+       return regmap_write(priv->map, RTL8366RB_PORT_ISO(port),
+                           mask);
+}

As sashiko pointed out, set_insolation(...,0) actually disables it
instead of completely isolating it.
I would unconditionally isolate the ports as we will never need to disable it.

Regards,

Luiz