I suspect there is no Undervoltage issue, all flags are set.
They are set before the driver is enabled the first time.
So, after the first power on, the Z axis is homed and there is the one-time report.
After homing of Z axis, X and then Y homed.
That is it.
Then there is no need for TMC autotune at all, it can’t be used.
I think it will not and the back EMF from the motor will have much higher value then that.
Anyway, there is no issue.
The drivers are cool, there is no errors, and everything works as expected.
Hope that helps.