Sorry that they are not listed in the first post, but Iris plus are rebranded Centralite plugs, and the general consensus is that Xiaomi / Aqara devices won't work when routed through Iris / Centralite plugs. I've updated the first post to add Iris to the list. For more details, do a search for "iris" just in this topic.
Unless you have an XBee and are using the XCTU software to map your ZigBee network, there is no way to know whether an end device is directly connected to the hub or is routing through a repeater.
You mention you have 30+ ZigBee devices, so you are close to or over the 32 end-device limit, which means even if you are pairing close to the hub, the device could still be forced to establish its connection through a repeater.
In my extensive testing and reading of other people's reports, not only are some repeaters incompatible in that they drop the connection of Xiaomi / Aqara devices, but also in some cases, messages from Xiaomi / Aqara devices aren't forwarded to the hub.
Well, a Hubitat hub is not the same as a SmartThings hub. The ZigBee hardware (and from what I understand the ZigBee software stack) of the Hubitat is different, so you can't expect everything to work exactly the same. The different signal strength of the Hubitat's radio alone means that the topography of the ZigBee network will also be different, because end devices will opt to connect to repeaters versus the hub based on signal strength.
The temperature / humidity sensor driver I ported over to Hubitat uses the same "fingerprint" data as in the SmartThings device handler to help the hub match the driver to the sensor being paired. I know this works correctly with dozens and dozens hours of testing with multiple sensors, both Xiaomi and Aqara, that I own.
If your sensors are being seen as a device then the data needed to match to the fingerprint must not be making it to the hub. This points to the possibility of routing through a repeater.
When you say that you see "null" in the logs after refreshing the device, I'm not sure what you mean. There isn't a refresh command built into the Hubitat driver. Also to understand better what's happening I'd need to see the log output.