Thanks for the report, Sandrinette. I assume the game you are talking about is 27049933, it has Forge and contains the situations you described.
What I think happened is that you got confused by the interface. Your first example seems to have happened at your turn 10:
Turn 10 - Sandrinette
S plays a Forge.
S trashes 3 Coppers and an Estate.
S gains an Estate.
S buys and gains a Copper.
S draws a Copper, 2 Silvers, a Witch and a Venture.
You played Forge, then trashed 3 Coppers and an Estate. Since Estate was the only 2-cost card in the game, the game automatically gave it to you (see the bolded part). When you thought you were selecting the card for Forge to gain, the game was already in the buy phase (your hand was empty), so you actually bought a Copper (as can be seen in the log).
The second example was later in turn 19:
Turn 19 - Sandrinette
S plays a Peddler.
S draws a Silver.
S gets +1 Action.
S gets +$1.
S plays a Forge.
S trashes an Estate and a Forge.
S gains a Platinum.
S plays a Platinum and a Silver. (+$7)
S buys and gains a Gold.
S draws a Silver, an Estate, a Peddler and 2 Ventures.
Again, since Platinum was the only 9-cost card, it was automatically given to you after you trashed an Estate and a Forge. Now again you moved to the buy phase, as you had no more Actions. If you have autobuy enabled, the game would now show you all the cards you can buy with the money you have generated ($1 from Peddler) and Treasures you have in hand ($7 from Platinum and Silver).
That makes $8, which allows you to buy the Forge you mentioned, but not the Platinum you were looking for at this moment. This is the same misconception as above, you think you are in the Action phase selecting what to gain from Forge, but are already in the buy phase buying a card. Unfortunately, the game interface does nothing to show you explicitely in what phase you are, making this misconception possible.