I've got a problem with my Pi3 B+ not reading my microSD card on boot (Red LED ON, Green LED OFF). The microSD I am using is a SanDisk U1A1-SDSQUAR-016G-GN6MA. I prepped the SD card with NOOBS and fired it up, and everything seemed to be working just fine. Raspian installed seemingly without issue, but when it went to reboot into the OS after install, it just never came back up. I get a solid red LED and no activity on the Green LED. I put the card into an older Pi A+ and it booted into Raspbian with no problems. So the SD card seems okay?
From there I tried a second Pi3 B+ to ensure it wasn't some kind of hardware failure with the first one. For this I did a complete overwrite format of the SD card with SD Card Formatter and then wrote the raspbian image directly to the SD card with Etcher. I wrote the June 27, 2018 image of Raspbian Stretch. On the 2nd Pi3 B+ it came up into raspbian with no issues. I configured some settings like connecting to the network and then rebooted via terminal command "reboot". The Pi shut down and never came back up. No boot, Red LED Solid but no action on the Green LED just like the first one.
Has anyone run into a problem like this before or know where I should check next? I don't understand why it would read the SD card with no problem the first time then have such a problem on reboot. I'm using a 2.0A power supply, which should be adequate to at least boot up [EDIT] I've hooked it to my bench supply now, which can provide 4A, so power is not the issue. Possibly an issue with the SD card? I don't actually see a listing for this specific card at https://elinux.org/RPi_SD_cards
[EDIT] I did some measurement of 5V and 3.3V rails and found that I get 5V (although a little funny looking, see image) and nothing on 3.3V. So not having 3.3V I imagine is a rather large issue. Any idea as to why it would just "go out" on two 3B+ boards after working on first boot?
[EDIT] I measured continuity with my meter, it's reading a short from 3.3V to GND (0.5 Ohms between them measured from the GPIO header). The question remains as to why this would 3.3V suddenly short off a soft reboot?