I’ve found the changes in the class-map to decrement the TTL for IPv4 traceroutes through the ASA however I cannot seem to find the same method to see IPv6 packets (ie, we’ve got IPv4 showing the ASA as the packet goes through it, but IPv6 does not exhibit the same characteristics, rather we get * * * all the time.)
For IPv4 we use the following (using global maps for easy of explanation rather than our uni-directional setup per interface):
policy-map global_policy
class class-default
set connection decrement-ttl
Doing a bit of research, others have also encountered the problem you are describing. The set connection decrement-ttl command does seem to only function for IPv4. I have been unable to find any solution for this particular case on how to implement this for IPv6. I will ask Rene to see if he has any insight on this.
One of the issues I did find is that the ASA 9.6.2 adds additional support for IPv6 that previous versions did not. Although I have found no documentation to support this, it could be that from this version onwards, support for IPv6 for this command may indeed be there. What version are you using?