In Mexico, I was told some on-demand h2o heaters have a small tank. My handyman said its because on-demand h2o heaters need electricity (or battery) to startup and we all know about CFE outages. No electricity = cold shower, so to get around that, some on-demand h2o heaters here have a small tank, enough for 1 shower. With Calorex, these are called "De Paso." Calorex on-demand with no tank is called "instantáneo."
Regarding the OP's question, also check flow rate of your Calorex. Brad Grieve has an article about this,, says a tub needs flow of 16 liters per minute:
https://www.mexconnect.com/articles/2951-water-heaters-in-mexico/
One problem I consistently see with homeowners who use demand-style heaters is this. The homeowner will use a higher flow rate of hot water than the water heater is capable of producing. For example, while filling the bathtub, the water flow rate was set at a greater capacity than the demand-style water heater could handle and the client was frustrated with cool to lukewarm water for their bath. We solved the problem by lowering the flow rate to a volume of water that could be heated sufficiently, slightly slower than the maximum capability. If the flow rate is not satisfactory, likely the demand heater size is not sufficient for its use. Typical bathtub users will need a demand-style heater with a capacity of approximately 16 litres per minute to handle the higher rate of flow to fill the tub.