Breath of dreams and Jade blossom (could be other older ramp cards as well, these are the ones I’ve encountered so far) don’t give you more mana crystals after 10 mana when you’ve played Wildheart Guff.
For Breath of dreams, the only thing that happens when at at least 10 mana, is that you draw a card. For Jade blossom the only thing that happens is that you summon a Jade golem.