CarPlay (and Android Auto, too) has a built-in limit on how many items it can display in a list. Apple Music, for instance, works around this by splitting the full list into partial lists, e.g., artists A-M and N-Z, with a button to switch.
I thought that ARC had adopted the same strategy a few updates ago:
Don’t you have this? Based on the timing of the above posts, I would think that it was included in this production update (which fixed a wide range of issues, though this one isn’t explicitly mentioned):