How should I handle a deck beam member that's slightly shorter than the others?
I don't like any of the A-C options presented here, the beam is a very important structural component and incorrect beam installations can compromise the entire deck. Shims, shortcuts and hoping for shrinkage are inappropriate. I would either:
D) Return/exchange the bad board for one that is proper size.
E) Trim the larger beam member to the same width (height, as installed) and adjust the post-beam notch or bracket to compensate for the slight decrease.
Of these two options, I favor option D but with sky-high lumber prices and low availability now (Summer 2021) that might be difficult.