I’m going through the same process and have experienced simular issues. In my case a brand new zwave plus s2 device was falling offline/ becoming non responsive about once a day after it got migrated to HE. Troubleshooting the issue has given me a greater appreciation for how crappy older zwave non plus devices actually are. I dont know why it wasn’t causing issues when paired to ST but after i replaced a very old non-plus device the rest of my zwave mesh has been considerably more stable.
If you have any zwave non-plus devices youre bringing over from ST you might want to consider upgrading them or kicking them back over to ST until you can.
My understanding: The 700 series chip certification process (the C7 was the first certified 700 series hub) introduced some additional requirements and constraints that the 500 series didn't have which affects the way the mesh is built and works. ST also used extensive polling of non-plus devices to overcome status reporting latency, but also may have had a slightly stronger antenna.
What you are describing sounds like a mesh problem, and generally can be overcome by adding a few Zwave Plus repeaters to the mix.
The Aeotec MultiSensor 6's (if that's what you have) will repeat if plugged in. Unfortunately they don't seem to repeat very well. There is an update to firmware you can add which may help but in general they should be on the periphery of your network if possible. I have 3 in use right now - one in the basement, one in the garage and one outside in an enclosure for lux testing only..