https://bugs.freedesktop.org/show_bug.cgi?id=109887
--- Comment #8 from Andrew Sheldon asheldon55@gmail.com --- I also can confirm the problem, and it seems to have gotten worse since 5.3.0-rcX.
In past kernels, you could kind of work around it by setting slightly less conservative undervolts and it would work. If you go past a certain point (a point that works fine if the pp table is overridden), it would wrap around to 1.2V. For reference this around 950mv at state 6/7.
Now with 5.3, even that same relatively conservative undervolt would immediately jump to 1.2V under load.
As hagar-dunor suggested, manually overriding the entire PP table, works around the problem.