Looks like a few 502 Tacticals got out of the factory with missing parts or parts not properly installed for the safety allowing the gun to fire when on safe
FN 502 Safety Bulletin | FN® Firearms
Inspecting the FN 502™ Tactical Manual Safety FN America, LLC, has become aware that a very limited number of FN 502 Tactical pistols in the field may have an improperly installed manual safety. In these cases, the external safety lever will not function as designed and may result in unintended...
fnamerica.com