Thank you, I have tested it and from the desktop wallet (where I've gotten the master public key) the answer is false, but from the "watch only" wallet the answer is yes. I don't understand anything.
Why the wallet created from the master public key don't shows the desktop wallet?
Why does it behave as if it were a new wallet but does not allow sending?
How can I find the private key or regenerate this "watch only" wallet to get funds from it?
By creating a wallet using the master public key, the new wallet will effectively become a watch-only wallet. This means that the funds can't be spent from the new wallet and you have to spend it from the wallet that you've gotten your master public key from.
Could you go to your desktop wallet and go to the console tab and type this, with the field inside replaced with the address you've sent to:
If the argument returns false, your desktop wallet doesn't have the address and you can't spend the funds.