Also posted the following at Routines not running and Unexpected Error in Routine when turning on multiple Arlo cameras
I got a response from support that they do not have an ETA on when this issue will be fixed:
Currently, we are suggesting users create duplicate Routines and placing only 2 Arlo Cameras in each Routine.
You can also try using Smart Lights to trigger based off of a Mode Change.
I tried the Smart Lights suggestion, which appears to have resolved the problem.