Since they both grant a circumstance bonus to AC, is it correct to say that if you have a shield (raised) you gain no benefit from lesser or standard cover?
Circumstances stack.
It makes sense that if an arrow has to go through a short wall AND your shield, that it would be a trickier shot than just one of the two.
And if you were raising your shield, juking back and forth like mad, AND hiding behind a waist-high wall, you should be even harder to hit.
Probably a good move from a rules-design standpoint.
I remember we used to do really exploitative Half-Orc Archeologist builds in PF1, and I'm assuming they want to curb that a little.
Also: no Luck bonuses?