It seems like noone used thumbnails in Holders before? My son needed this to hold levels for a game. Since each level is almost full-screen, thumbnailing them was the only viable solution I could think of. So here's a one-line fix to make it work:
"Change Set: ThumbHolderFix-bf Date: 30 September 2002 Author: Bert Freudenberg
Makes getValueAtCursor and getGraphicAtCursor for a Holder showing thumbnails return the represented morph. Before, the thumbnail itself was returned which caused the animation to be played at thumbnail size."
-- Bert