I have been experimenting with the USB UART click board included with the click mezzanine starter kit (AES-ACC-U96-ME-SK) and was surprised to see that the USB UART/I2C click board appears to be supplying power to the Ultra96 when connected to a powered USB cable, but when the main 12v barrel jack supply is disconnected. It is possible to power on the board with only the click board USB connected (no 12V barrel jack) though the boot does not complete, presumably due to some detection of low power. Furthermore, the VIN_LED, also lights up when ONLY the click board USB cable is connected (no 12v power supply) although noticeably dimmer.
Looking at the Ultra96v2 schematic, this behaviour appears to make some amount of sense, as the 5V output on the low speed expansion header is directly connected to the main 5V rail that feeds into the "Rocky PMICs". However what I cannot explain is why the VIN_LED, which is supposed to indicate that 12V is connected to the board, would light up when only the 5V USB is connected. There doesn't appear to be any direct connection between the 5V rail and the VIN_LED based on the schematic.
The Ultra96 HW user guide does not mention anything about supplying board power with the 5V pin on the low speed expansion header. Is this expected behaviour, or is there potentially something wrong with my board?