iPad Air 4, up to date. Sandbox. I find my builders will stop doing any jobs when (I am guessing on this) there is a too long queue of jobs to be done. They just wander around daydreaming (etc) unless I actively prioritize a job (beyond the default priority from the order I assigned it to be done).
I am not sure if this is all the time or just sometimes, nor what is the threshold of jobs-to-be-done. Mostly I am focused on the master plan and it gradually dawns on me that my builders are making zero progress. I am currently only playing sandbox, (long since done with all levels) — I am not sure if it happens in the classics or various chronicles.
Most recently, it happened when I had a queue of 12-14 jobs. I will pay attention and see if I can narrow in on why they stop working.