I think I am safe assuming that as you only bought your car 7 months ago it has had least one previous owner or maybe more. Hopefully at some time in it's life it will have gone into a Mazda dealership for a service or warranty issue. If it has in fact done so, after the issue with the improperly hardened camshaft head was recognised, then it should have been checked. Exactly what was involved regarding rectification I am uncertain. I do know that it involved a complete engine flush and oil replenishment.
If I were you, my immediate task would be to contact your nearest Mazda dealer armed with your VIN number and get them to check on their database to see if your car has been checked. Your next action can only be based on the result of that enquiry.