Event field mapping rules are not working when we map the source to a different value other than the event source.
Event Compose rules always get executed before field mapping rules. If you are mapping the source value from compose fields, your field mapping rule source should match the source value after compose rules are executed.
From your event rule, if you are mapping source to value 'OEM' as opposed to the actual source of the event. Your Field mapping rule source should be 'OEM' as opposed to the source of the event. This is because compose fields always get triggered first and we change the source to OEM, not we look for all mapping rules with source as 'OEM' as opposed to source of the event that is coming in.