I'm getting similar experience to MikeyFresh. The IdPi referred to below has the following Version/Build detail:
Id:24.0.4 MC:24.0.25 Engen:1.0.69
Today, I just got a 5.1V 2.5A official Raspberry Pi Foundation OEM power supply, noted as being suitable to power the RPi 3B+. I got an unfriendly response on screen (refer 1st pic) with the lightning bolt (undervoltage indicator), when I booted the 3B+ IdPi.
I also took delivery of a USB Power Meter:
https://www.adafruit.com/product/2690with claimed specs of:
Specifications:
USB 1.x and 2.x - all data speeds. Does not have USB v3 pins so USB v3 connections will automatically downgrade to v2
Working Range: 0~3A, 3.7~13VDC
Resolution: 10mV / 10mA
I swapped to the Logitech PSU that I've got that's rated to 5.1V / 2.0A as noted here:
https://yabb.jriver.com/interact/index.php/topic,115740.msg800304.html#msg800304because it allows me to get an easy read on voltage and current (USB-A M-F connections on the Adafruit OLED V-I USB meter) and I know that the PSU works with an IdPi on a 3B running a Digi+ Pro or a DAC+ Pro.
Firing up the 3B+ IdPi with the Logitech PSU gives me the 2nd pic - where the current draw maxes out at 0.17A at 4.98V. It just flat lines on that low current draw and the indicator lights stay red and do a repetitive cycle from blink to solid. Note that the IdPi is a bare unit without ancillaries hanging off it, e.g. HATs, Z-wave sticks or USB drives. The screen displays the same splash screen with lightning bolt as referred to previously.
Using an instance of Volumio to test the bare 3B+ (refer 3rd pic), the 3B+ comes to life, boots and stabilises to draw 0.51A at 5.00V. I did this to confirm that the 3B+ RPi hadn't been fried by static or some such disaster and thereby rule out that possibility from the troubleshooting list.
From previous tests, I can serve audio from MC to Volumio as a renderer and it functioned happily with no protest and was carry a HiFiBerry Digi+ Pro as well. I haven't got V-I info but if required that can be done.
Finally, I moved the IdPi card back to my 3B setup that includes, and it was powering, an Aeotec Z-wave stick and a Digi+ Pro HAT (refer 4th pic). In this case the IdPi idles along once it has booted up to draw 0.33A at 4.94V. Note the IdPi wasn't serving audio at the time but I know it can handle the associated load (whatever it is).
... and that's about it.
Final point - If I had suitable adapters, I'd check the OEM PSU and the iFi PSU (note this PSU hasn't been able to motivate a 3B+ IdPi either, although that test was done with an earlier build) that I've got in the house and take V-I readings.
Thoughts, feedback, ideas?