I was attempting to change all my railing parts (spindles, toerail, handrail) at the same time by using the "Edit Custom Assembly" in the Railing Edit Dialog box. I am using a custom rail finish ("Railing finish 10") and I did Surface Edit/copy from another material in the model to get the color I wanted for all the railing parts (if that matters).
The Issue is that while all the rail parts are shown to be selected in the "Finish" drop-down ( in this case, "Railing finish 10"), the posts & toerails change to the desired finish, but not the spindles (see attached screenshot)
Anyway, if you do the following , perhaps you get the same result::
right-click on Railing>Edit>Edit Custom Assembly>click on "Finish" column name>select "Railing Finish 10" from drop-down>all rail parts change to "Railing Finish 10" in the dialog box, but the spindles finish doesn't change in 3D view