Is the “power supply” a cheap voltage controller used for heating? That’s what I suspect, and it is of the “zero cross variable time base” variety. That can only work on purely resistive loads because it is always putting out full voltage, but is simply varying the number of on and off pulses to deliver an AVERAGE voltage to a resistor. Each pulse however is seen by the capacitors as the full RMS and stored, hence the full voltage measurement and the line terminals of the drive.
If you used this in the past to reform drives, you got lucky because this type of power supply is a capacitor killer, but only IF your caps have already de-formed. If they were OK to start with, they won’t be harmed.
If you are going to use an electronic AC power supply (as opposed to a manual variac), it must be the type called a “Phase Angle controller”.
I’ve also stopped doing this through the AC input terminals because in a couple of cases slowly raising the AC input caused the brains to get scrambled. I’ve taken to removing the terminals of the caps and connecting variable DC directly to them.