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.