I ended up finding my Windows 10 Live HDD and used that to boot. It's Win 10 Enterprise (Evaluation).
This test makes me even more confused. Upon first boot, I plugged in the iPhone XS and it installed the driver as expected. Data and power were working. This was in the top left port. I then plugged it into the bottom left port, but this port would only power the device (no data). I unplugged it 10-20 times, and there was one case when I was able to get it to connect with data, but it was brief. The next time I reconnected it, it no longer had data connection (just stable power). Never once did it do the disconnect/reconnect thing. The top right port was occupied by the USB HDD.
I rebooted into MacOS to see how things were working. The top left port continued to display the disconnect/reconnect symptoms. This port worked perfectly on Win10. The bottom left port was doing the same thing as Win10 - power only. But it was consistent. The top right port was doing the disconnect/reconnect dance.
I booted back into Win10, except this time I used the top left port for the USB HDD to boot from. The bottom left port did the same as before - power only with data in some cases. The top right port this time worked perfectly - always giving me data and power each and every connect.
Back to MacOS one last time. I plugged into the top left port, and to my surprise, I have a stable data and power connection. Here's what it shows in the USB tree when it's fully connected successfully:
View attachment 415892
Notice we now see the Extra Operating Current and Sleep Current reported correctly. The location ID is different too (even though all of these screenshots I took are from the top left port).
The bottom left port is still only giving me power. The top right port is doing the disconnect/reconnect dance. I just plugged it back into the top left and it seems stable again.
All tests were done with the same exact USB A to Lightning cable and all were done with the iPhone at around 70% battery (obviously it went up as I was testing). The computer was at around 80% and was on battery, it ended around 70%. I notice that there isn't much of a difference on battery vs. power.
It seems that the bottom left port is the "worst" port to use on any OS. The top left one seems to sometimes work in MacOS, but always work in Win10. Top right I haven't gotten to work on MacOS but it works consistently on Win10. Remember that this is ONLY with devices like the iPhone XS - USB drives and older iPhones work on all ports, all the time (both data and power). Since I'm experiencing some weirdness in Windows 10 (but less so than in MacOS) I'm guessing this is some sort of hardware limitation I'm hitting.
Any ideas?!?