Hard to tell, it may have to do with your zigbee coordinator or the number of repeater devices in your network.
Which coordinator do you use?
If the network is not well meshed then the link quality could be too low for the sensors to reliably stay online. Adding repeater devices (mains powered devices like bulbs) could help here. Or if you have too many devices your coordinator may be overloaded. I had this problem for a while where I basically had to restart the coordinator because every device was offline. This happened once or twice a month. A firmware update helped here.
Generally zigbee2mqtt is superior to ZHA in my experience, but a little more work to get running. But you will find lots of documentation and YouTube tutorials on how to set it up. Not sure if it will help if your network is “weak” though.
But even if your zigbee network is great there are some devices that are just shit. I have a few analog LED controllers that randomly drop off the network and will only rejoin after cutting power to them. Doesn’t matter how good the link quality is, they go offline sometimes.
So maybe the soil humidity sensors are just not good?
Well, the coordinator is probably not the problem, and the soil sensors are probably fine, too; from what I have heard Thirdreality devices are generally quite good. I trust you are using a USB extension cable for the coordinator and don’t have it plugged in directly in the USB port?
No idea how good the smart plugs are, but if one of the soil sensors is basically next to the coordinator and still falls off the network randomly then the problem is likely not the plugs or the network mesh.
All things being equal I would suggest switching to zigbee2mqtt and see if that helps. Even if it doesn’t, and the culprit is something else, zigbee2mqtt is (in my experience) better in the long run, because it generally supports more devices than ZHA and is much quicker in getting new devices supported.