As @Tibo pointed out, this is how the rewards work for Dungeons and descending from the updated patch notes
Mini Bosses will get:
- +1 Shard every Level
- +1 Jewel every 4 Levels
- +1 Diamond every 20 Levels
While the Mega-Boss will get:
- +2 Shards every Level
- +1 Jewel every 2 Levels
- +1 Diamond every 10 Levels
Once you have reached level 10 if you have not seen an increase in the Mega Boss Diamonds please reach out.