The plot thickens. The test described above was powered by a solar usb power bank directly into the wemos. Now, if I powerthe wemos from that same power bank it refuses to go into deep sleep. I measure 4.7v at the 5v pin on the wemos while it is awake.
Now here's the mystery.
If I run the wemos from the laptop usb port, it runs perfectly, albeit short term. I again measure 4.7v at the 5v pin when the wemos is awake.