I have 52 Zigbee plugs in total. 1 Iris v2 Beta, 1 SmartThings v2, 2 SmartThings v3, and 48 Iris v2. There are also 4 Halo smoke detectors, 1 ThingShield, and now 2 Xbees. All are acting as routers according to XCTU.
Do you have the latest firmware version in the Iris plugs?