After adding a slave to Master - Master configuration, slave syncs from only one master
You need to ensure you have log_slave_updates enabled on M1, so that any changes that M1 makes via replication from M2 gets written to M1's master log (and thus replicated to S1).

Don't worry, the entries in M1's binary log will not get re-replicated to M2. M2 will ignore any updates which originated from its server-id.