rp= u/gm, both u and gm constantly change during a cycle. Both changing independently, not in sync, in value and causes rp to rise.

This can be graphed in a quadrant. The Ra curve (at different plate voltages) rises gradually at first over the first 5 volts peak or 10 volts p-p; then rises rapidly. Even then, a 10 volt peak, 20v p-p, drive signal can easily cause Ra to change by 20% or more over a cycle. The slope of the loadline is very important. A more vertical loadline dramatically increases Ra's change and effect. A more horizontal loadline helps minimize, or even eliminates Ra changing. But a horizontal loadline causes the output power to lower, rather dramatically.

This applies to any triode run SET mode. Just something to think about when designing an SET amp. RL is important.