No, it will be a problem. Not only the LTC3245, but all other semiconductor devices have a common GND internally, but since they are connected in a state of high impedance, be sure to connect the GND pins together outside the IC. In the case of the LTC3245, the Exposed Pad (Pin 13) is for heat dissipation, so if left unconnected, the thermal resistance increases and the junction temperature rises due to insufficient heat dissipation, which reduces the maximum output current relative to the ambient temperature. is expected, and the return current path of switching is also weakened, which may lead to an increase in noise.
On the other hand, if the pin 12 GND is open, the signal system reference will be weakened, and depending on the input/output conditions and layout, unstable operation such as unstable output voltage may occur.