Yes, I did try it, but what happened was it severely reduced the DAC's capabilities (limiting the sample rate to 96kHz, and possibly the bit rate down to 16-bit, but I need to verify that when I get home from work) instead of simply disabling the volume control. You also mentioned that the E50 was UAC1.0, and that's why the volume control was disabled with that DAC connected to my Mac, but I don't see anything in the manual indicating the USB version of the E50? The only difference appears to be the connection used (USB-B vs. USB-C) as the input range in both manuals are identical (the D50 III has the added line regarding PEQ, but everything else is the same).
It seems like it's something else to me, but it's also not obvious what would cause the difference in that specific functionality. When I was using a D10s DAC, it would sort of work in the same way the E50 worked with the volume control, but sometimes not (it would change from time to time whether it wanted to disable volume control), and when I tried out the MX5, it functioned the same as the D50 III.