I tried the longer cable a second time with an inline amp with the same result. After a second restore, it functions normally again.
The cable is HDMI 2.1 and has worked fine in other applications. But, to me, the mystery lies in the fact that the change in the nuc remains when I put the shorter cable back. At present, I've put the Intel nuc aside and am using a HP nuc to work on my project. When I'm finished, I'll go back to this problem and give the Intel technician what he's asking for.