So the tube has an internal bias circuit that could be malfunctioning and the tube would still test good? The bad tube shows no bias in any of the 4 sockets, and a known good tube biases fine in all 4 sockets.
Mysterious power tube behavior
On my Willsenton R8, I was using JJ 6L6GCs. One of them got a bit noisy, and then soon after startup the bias would go to zero; bias in the other positions remained steady. I switched it to different sockets, same behavior. I threw in a set of known good KT77s, all 4 positions bias fine. Put the 6L6s back in, that one tube still caused bias to go to zero.
The mystery is that sound quality, other than the noise, was never affected. And the misbehaving 6L6 tested fine on my recently calibrated Hickok 600; the reading were exactly the same as the other 3 tubes.
Anybody got any idea how a tube with zero bias could act perfectly normal? Visual inspection of the tube didn't show any differences either. I've been using tubes a long time and I've never seen this.