This is normal behavior. The ASA is a security appliance and therefore "stealths" itself. It does not decrement the TTL therefore doesn't appear in the traceroute. You can have it appear in the traceroute by using the following if desired.
ciscoasa(config)#policy-map global_policy
ciscoasa(config-pmap)#class class-default
ciscoasa(config-pmap-c)#set connection decrement-ttl
