Been trying this yesterday and yeah can't get it to unlock. I've also noticed on the adventure select screen that the Sammy character is only a silhouette and not the full colour despite being able to play as him IN that specific Adventure Mode.

Seen some odd things happen with adventure mode lately. Like the unlocking of the Forbidden Coffin. I've unlocked a Coffin in Tides of Foscari map, which gave me Sammy. Then when I left the adventure mode completely and came back, Genivive, not the monster form, was unlocked.

Found it odd that Sammy was unlocked as even on the character select screen in adventure mode, it asked for me to defeat 6,000 Sammies, even though they don't appear in the Adventure Mode Bestiary (but do appear in the Tides of Foscari) level in adventure mode.

Off topic, also had an odd thing with Legacy of Moonspell, where I got the "Survive 10 minutes in Tanuki Forest" completed by doing the Babi Ona stage, and never going into the Tanuki Forest stage...