Update:
Yesterday, they noted that they thought it might be in an instruction that couldn't do some predictive thing (me not programmer). It was a vague comment, not intended for action, but we thought we'd take a spin and remove that instruction from the shader. It still had the problem.
Today, they asked for an extra day because they had a fix, but they wanted to "double check" it.
Whether this fix is information on changing our shader usage, or a new driver, I don't know.