Most likely because your water flow is too strong for the water heater to handle.

Your instant water heater's output water temperature is affected by:
1. Inlet water temperature | The lower the original water source's temperature, the lower the output water temperature.
2. Power rating | The higher it is, the hotter the output water temperature (but higher electrical components' specs required).
3. Water flow | The higher it is, the lower the output water temperature.

If I am not mistaken, your Toshiba model (and most, if not all Toshiba instant water heaters) are only 3.8kW. In comparison, other brands like CENTON, DEKA, RUBINE, etc. are higher, in the 4.xkW range.

And if your water flow is high, your Toshiba water heater will not be able to achieve the temperature that u want, even at maximum capacity. Hence, the blinking is to notify u that it is already at this maximum capacity and cannot achieve the temperature that u want.

If u want to achieve higher output water temperature, try lower the water flow and the output water temperature should increase slightly.