Hi!
Apart from the hassle of arguing it out with his original Supplier, with all the aggro of waiting for a replacement to be supplied, the OP could well do that and find out the replacement that comes has an even worse overshoot, and the Supplier/OEM denies it's a design fault, which it most assuredly is!
The reason a turn-on overshoot occurs is because the control error amplifier is operative before the rest of the PSU has started to provide a feedback-voltage to it's voltage-control input.
By putting a time-delay on the reference voltage for the PWM controller, at the moment of power-on, the Ref. volts will be at, or near chassis or primary 0V potential, so the PWM control amplifier will give maximum negative-feedback control output at the instant of power-on, helping to prevent an unwanted high-voltage surge from the SMPSU's power-transformer.
As the reference voltage builds up from chassis (or 0V primary potential), the PWM Control Amplifier's negative feedback will reduce until the normal amount used in the circuit design is reached, by which time the output from the SMPSU power transformer will have built up enough for the output-voltage feedback loop to take over.
I have attached some notes from Keith Billing's "Switch-Mode Power Supply Handbook" which discusses the theory and gives a simple practical circuit the OP can try, and it works if the Control Amplifier is part of a '494, '3524, 741s, discretes, etc., etc., as long as both it's reference and error-voltage control I/Ps are accessible, in either linear or SM-based power-supply units!
If the OP opens up his unit and advises us what PWM controller is in use, we can suggest a circuit idea for him to try! I've assumed, from the OP's description of the overshoot-spike, that his PSU unit is likely to be a switch-mode based one - a turn-on overshoot spike of 70V certainly suggests so!
Chris Williams
PS!
I believe Siglent had a nasty issue with turn-on overshoot with one of their models and had to cajoled into developing a cure for it!
https://www.eevblog.com/forum/testgear/siglent-spd3303d-review/