Just wanted to add to this thread… I installed a new Red Dimmer tonight, and it’s experiencing this problem (lights flipping off after a few seconds when the dumb three-way is on one side, lights are fine if it’s on the other side). Resetting the settings through SmartThings as described here didn’t fix the problem.
For the moment the problem seems to be mitigated by setting the max brightness to 90 (I actually stumbled across that fix on my own prior to finding this thread, thinking maybe the switch was going into some sort of overload protection or something).
Setup has a single dumb three-way switch, with neutral. Lights are 3 40W G9 halogen bulbs and a single 9W dimmable LED (and I did try taking that LED out of the picture, thinking it might have been an issue with two different types of loads; didn’t help).