Energy Reset in Deck Builder
There are a few examples of this issue triggering, and both seem to stem from the same function. The issue is: when editing a deck that uses energy that does not match energy requirements of the card in the deck, the energy is automatically changed without the user's consent.
Example 1: Deck uses cards with more energy types required for attacks than energy assigned to the Deck. Dragon decks are a great example of this because many of them use multiple types of energy, but not all energy types are necessary to use the Deck. Cards like drudigon are often used as defensive cards with no intention of attacking. Because of this, fire energy is often excluded in building the Deck to boost efficiency of the other cards in the Deck. However, if you go back to make changes to a Deck like this, the fire energy that was originally omitted is automatically reapplied to the Deck without notifying the user. Obviously, when you go to use the deck, you are immediately confused and put at a disadvantage when that fire energy spawns.
Example 2: Decks without energy type requirements are automatically changed to grass energy type. This is not usually a hindrance in attacking with a Deck of this design, but can conflict with item or support cards that rely on specific energy types like Irida (requires attached water energy) or the new item that moves a lightning, fire, or water energy from a benched pokemon to the active pokemon. A colorless Deck could leverage both of these cards with a water energy chosen. However, when you edit this Deck, much like the previous example, the energy is reset to only a grass energy instead of the originally chosen water energy. In this situation, you will draw cards that are entirely useless because your energy type was changed without you knowing.
Both of these examples give a competitive advantage to the opponent in the first game the user encounters this issue. I also do not think this is a bug, but rather an oversight by the developer who implemented this automated energy choosing function. In both cases, the function is meant to cover for an assumed mistake by the user, but the game does not take the nuances of these decks into consideration. My suggestion is to remove the energy auto-choosing altogether, especially in instances where the Deck is being edited rather than initial building. A secondary fix would be to only implement this feature on net-new decks, so that energy is not changed when going back to edit an existing Deck. My final suggestion would be to assign a hidden value to support cards that interact with specific energies, though this only solves one of the two examples of issues with this feature.
Thanks for your time and I hope this was clear / helpful.