Welcome to MongoDB community!
Starting MongoDB 3.6 change streams are using read concern “majority” by default which means that only majority node commited data will be streamed.
When your PSA replica has only one data node there is no majority commited data therefore it won’t stream
The good news for you is 4.2+ changestream has disabled the required need for read concern majority but you have to disable this default behaviour:
Btw its a recommended change for better stability for PSA replica sets anyway.we recommend moving to PSS and replacing arbiter with a secondary for overal rs HA…
Thanks
Pavel