The CI Relationship record's Relationship Type reference field (cmdb_rel_ci.type) has a bad default value of 1, allowing inserting records with broken references.
There is obviously no cmdb_rel_type record with a sys_id of "1", so this becomes a broken reference on inserting a new record without selecting a type.
Although a cmdb_rel_ci record makes no sense without a Type, the form does not make this mandatory or prevent you doing this.
Steps to Reproduce
Open a blank CI relation for m - /cmdb_rel_ci.do
Fill in a parent and a child, but don't touch the Type field (which will appear empty).
Check the XML and you will see the type=1, which is taken from the default value in the dictionary
The value is empty and remains empty, or starts empty and is made mandatory.
This fix from London onward only applies to newly provisioned instances.
The workaround for existing customers upgrading past London is to simply clear the default value in the sys_dictionary record for table cmdb_rel_ci and field type.
Related Problem: PRB1251706