Shader nodes must have unique names.
So if you’re using standins, each standin ASS file has to have unique shader node names. Otherwise you’ll get unexpected results when you render, like all standins having the same shading, or some “flickering” in animation if the order of standin loading changes.
In the Arnold log, look for “node name already in use” warnings:
WARNING | [ass] standin_01.ass line 50: node name "aiStandard1SG" already in use WARNING | [ass] standin_01.ass line 58: node name "aiStandard1" already in use WARNING | [ass] standin_01.ass line 118: node name "file1" already in use WARNING | [ass] standin_01.ass line 147: node name "aiNoise1" already in use
If each standin should look different, you need to have unique shader names in each ASS file. If you’re using MtoA, that includes the name of the SG node (for example, aiStandard1SG).