Weird battery and throttling bug with UX490UA (Zenbook 3 Deluxe)

Bought a refitted UX490A with i7-8550U, perfectly fine for two months but all of sudden (after charger plugged in for more than a week) the speed was reduced to 400MHz, can't go up no matter what I do, even for benchmarks. And random sleeping on Windows 10 until I removed ASUS ATK program - I saw in event log sleeping was triggered by "application interface", apparently not a system function.

The speed issue affected both of Windows 10 and Ubuntu 18.04 - tried all CPU governors and changing min frequency as well. In event log I saw "The speed of processor 7 in group 0 is being limited by system firmware" at every boot. And charging always stopped at 50%.

Then I let the laptop to discharge to 12% and shut down automatically. Boot again, speed is now restored, still charging.

If it's not caused by new Intel microcode, it has to be a bug or some setting in BIOS related to battery/charging, that made the laptop to think the battery level was too critical for any speed above 400MHz, but then it can't explain why charger was unable to go beyond 50%. Anyone has any idea how to find out such setting in firmware, or alternative/custom BIOS?

 

Reflashed same version of BIOS, no apparent effect.

But then I was able charge from 6% to 100% while powered down. Maybe Windows and Ubuntu were affecting charge somehow?

Everything is normal again, for now.