The current (common) rpi3_pm.c code doesn't implement a .pwr_domain_pwr_down_wfi callback to power off the cores. Of course, we can't really power off the cores or anything, but we should be putting them into a holding state to wait for another warm boot via PSCI CPU_ON.
This affects ACS tests (which use PSCI CPU_ON repeatedly) and also the ability to offline/online code within OSes like Linux.
I have a patch against @pbatard's tree, since that's what we're shipping today as part of https://rpi4-uefi.dev releases.