At the moment, when tabs hide their components are removed, so when you go back to a tab, it initializes again - for example, the OnInitializedAsync event fires where you fetch data, so you have to fetch the data again.
This behavior should be controlled through a property so one can keep the original behavior where the components are cleared from the DOM and memory.
Perhaps an option can be added where tabs are initialized not on initial load, but upon first activation only. Maybe the feature can become something like an enum - LoadMode="InitialRender|FirstRender|EveryRender" (working title off the top of my head).
At the moment, a solution you can use is to send the data to the tab from its contents through an event, and to pass it to them as a Parameter so you can check for it before fetching data.