Update - I tried changing power supplies on the chance that the Express actually was actually trying to draw more power. The brick that came with it was rated at 5V/1A, and I switched to a 5V/1.55A brick I had for my cell phone, which works fine. The low power warning continued to occur (and the draw when it is in the error state is 5.04V/.68A, well below the rating for the power supply. I also disabled adblocking to see if there was an issue there.)
This reinforces the theory that it is a software problem from the most recent update.
This is extremely frustrating, and while I did receive a reply that the support team will investigate, I don't hold out much confidence that this will be resolved - given the forum history, this has been a known problem for quite some time, and no resolution has been implemented. What needs to happen is that a means is made available to revert this to an earlier software version, but as with any closed system, that supposes that Roku is willing to do so, which I doubt, given that they don't even provide support options for these earlier units.
I have ordered a Firestick, which arrives early next week. If that is reliable, this will be my last Roku, and I'm not going to be recommending them to anyone. Even the Firestick is a gamble, since it is yet another closed system. If I have problems with it, my next route will be an unlocked Android box or Raspberry Pi that I can control the builds on.