On some view call to get ft_balance_of() some account, I received the following response:
Log [develop.aurora]: Balance of NEAR [testlocal.testnet]: 29977777805780128
Account balance of testlocal.testnet: 29977777805780130 yoctoNEAR
The first line is the log from the contract, the second one is the response from the function itself that is being converted to ethers.BigNumber.
The glitch is quite small, though it makes sense to get the reason for that and solve it.