MySQL Master-Slave replication foreign key constraint error
I see that you have used a multi-master configuration for m1 and m2 and a master-slave config for m1-s1 and m2-s2. Currently, the multi-master configuration does not support write actions on the tables with foreign keys. Thus, when there is a write action on either m1 or m2, the error occurs.

test to debug: set up a master-slave config for m1-s1 and m2-s2 without connecting m1 and m2. If both of them work as expected, then you can confirm that the foreign keys issue is due to the multi-master m1-m2 config.