I'm using a Pi Zero W, testing on many power sources. I've testing all power sources with a multimeter and have confirmed that they are all 5V (+/- 0.1) and have adequate current available. 2 amps or more.
When I connect to a "bad" power source, HDMI screens get either flickering or no signal. Connected to a "good" power source they work as expected. With all power sources I'm able to SSH into the pi and do whatever I want.
Power sources that work: Wall wart, USB
Power sources that don't work: Anker power bank, Flepow power bank, 6x AA batteries in series with buck transformer down to 5V, 8x AA batteries in series with same setup.
I've found zero helpful info online and I'm stumped. My first guess was a bad power supply so I dug out a ton of batteries to test both name brand and import.