The Festive Mash-up world template is missing music discs in many chests in the locations you can explore in the pre-built world. Also, many of the mobs that are supposed to be present in the template such as iron golems and villagers at spawn as well as other nearby locations didn't spawn as they should when you create the world. Instead, monsters and dropped items are littered across the world which is easily noticed during the first few seconds of exploration. This wasn't a problem with the console editions. Again, you can see these irregularities by roaming around this world in creative mode and check the chests where the music discs are supposed to be in and the various locations in the world template. I don't know about other Mash-up pack templates but last time I checked there was a similar with the Fallout world template so please check that one too (I can't check since I don't have it right now).