Bill Tindall, E.Tn.
...to those that have experience on many samples of lumber, in my case walnut, but it could be other domestic hardwoods.

Can one run into grain that just won't plane without grain lifting from what seems to me to be a modest upsloping grain orientation?

The answer is of great importance to me for I am at a fork in this road. Do I write this experience off as an unusual occurrence and carry on? Or, does there remain a flaw in my preparation or technique to be corrected?

The blade is near freshly sharp, the cap iron set close enough for every other grain reversal encountered, the shaving thickness is thin but not as thin as practically possible.