I know this is kind of an old thread, but I am having the same issue with the NanoPi Duo2. When I install the latest community build based on 6.12 kernel, the reboot just powers down the unit. I tried the legacy 6.6.75, and this also has the issue. The only way I could restore functionality was by going back to a 6.6.16 kernel build, and disabling kernel updates in armbian-config.
There must have been something changed in the way the kernel issues the reboot command that the H3 is not interpreting correctly. If anyone finds a solution such as a kernel argument that fixes this, please share as this will prevent any further kernel updates until fixed.
