The 410c Board is designed to deliver 1.18 Amps from each of the Type A USB 2.0 ports. The current is measured and limited by the TPD4S214YFFR chips on the board. We intentionally set the current limit very high because we know from (bitter) experience that many “USB compliant” devices draw far in excess of the 500mA limit. The remaining power system on the board (the 12V to 5V stepdown) has also been designed to handle the worst case where both USB ports are drawing 1.18 Amps, all while the mezzanine card is also drawing the max possible current.
A “compliant” USB device should only draw 100mA during the enumeration phase, and then should request permission to draw more current, if permission is granted then the device may draw up to 500mA. The Type A ports on the 410c don’t limit at 100mA or even 500mA during the startup and enumeration phases. If the device draws in excess of 1.18A (for example when first plugged in to charge the capacitors) The TPD4S214YFFR will limit current, but it will retry again almost instantly.
The micro USB port (J4) is a different beast. It actually enforces the power rules when you plug in a device. If the EasyCap is drawing 150mA without requesting more power then it will get shut down. The current measurement mechanism is not really accurate, so just the differences between individual DragonBoards may explain why it works on some boards.
The micro USB port on the DragonBoard is supposed to be a “Type B” port, it is not a “Type A-B” port, you can see pictures here: https://commons.wikimedia.org/wiki/File:Micro-USB-jacks.jpg. A “Type B” port is intended to be a device, not a host. You must be using an unusual cable that allows you to connect a device to the micro-USB port on the DragonBoard. As you have seen, it will work, but that was not the design intent.
Is there any reason why you are not using the Type A ports?.
Full Disclosure: I am an employee of Qualcomm Canada, any opinions I may have expressed in this or any other post may not reflect the opinions of my employer.