Why does Quantum Calibration mode, Scanning mode, Mining mode, and any other sub-UI mode, take away or hide crucial flight information such as speed and altitude?

They were basically developed under the hood as "different contexts", and in their overhaul of the design they're factoring in all of the flight information so that it'll be available to Players regardless of what mode they're in, because it's still relevant when they're still flying, and therefore the information should be retained. In these modes referenced in the question, they're looking at potentially contextually changing out the "screens of cells" so that rather than the HUD changing for the different modes, they can shift elements around so that relevant information can still be displayed, rather than the new HUD for that mode just taking up the whole screen. Jared adds that it wasn't intentional to take away crucial information when using these modes, and that it's just something that's happened over the course of development that needs resolving. Simon adds that sometimes you don't realise it's going to cause issues until you try it, and this is one of those situations. Jared goes on to say that things can be developed in isolation, and then when they're integrated together into their game-dev branch, that's when they can see collisions and thus the creation of bugs.