As of r26913, it looks like the Hallowiener Dog adventures are no longer changing the lastEncounter preference.
I confirmed that the preference *is* getting changed in r26912:
I have also attached a debug log of the choice adventure, if that helps. I looked at my log and didn't notice any other kinds of adventures that were failing to change lastEncounter. Let me know if I can help to provide any other info.
[837] The SMOOCH Army HQ
Took choice 1106/3: acquire familiar food
choice.php?pwd&whichchoice=1106&option=3
You acquire an item: Ghost Dog Chow
Preference _concoctionDatabaseRefreshes changed from 2278 to 2279
I confirmed that the preference *is* getting changed in r26912:
[830] Pirates of the Garbage Barges
Preference lastEncounter changed from funky pirate to Wooof! Wooooooof!
Encounter: Wooof! Wooooooof!
Took choice 1106/3: acquire familiar food
choice.php?pwd&whichchoice=1106&option=3
You acquire an item: Ghost Dog Chow
Preference _concoctionDatabaseRefreshes changed from 2256 to 2257
I have also attached a debug log of the choice adventure, if that helps. I looked at my log and didn't notice any other kinds of adventures that were failing to change lastEncounter. Let me know if I can help to provide any other info.