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On minting single or two NFTs phantom wallet shows the balance that will be detected before signing Tx but on multiple mint more than 2, it will show msg "unable to fetch balance changes" but it will mint all NFTs.
But if I view advance transaction details then it shows me the transaction details and amount that will be detected for each NFT but not as a total in "Estimated Balance Changes".
TLDR: You likely aren't doing anything wrong here. This is a limitation with the current state of simulations and we are working on a more robust solution.
Phantom simulates transactions via Solana web3js's simulateTransation method. Currently, when multiple instructions and/or transactions are present, Phantom will add all instructions to a single transaction and attempt to simulate them together. We do this because there is currently no simulateTransactions (plural) endpoint, and we need to show the result of applying a series of transactions to the current state of the chain.
This works great when all instructions are able to simulated within a single transaction (things like 1 or 2 NFT mints). However, as you noted, this fails when the payload is too large. We don't batch requests to send multiple simulateTransaction requests because each transaction would be simulated independently on top of the current state and then reverted before the next is simulated. This would not show the true result of approving the transaction.
We're in the middle of upgrading our simulator across the board and will soon have a better solution for properly simulating these large transactions!
