When you hold an underground belt/pipe, it correctly shows the range just as expected.
As soon as you place the first part of the underground unit down, that range indicator now disappears and starts to move as your move the mouse cursor, holding the "exit" part of the belt/pipe. So you no longer see what the max range is because it moves with the mouse now instead of staying in place to show where the "exit" should be placed for maximum range.

If this is intentional I kindly ask for an option to disable this behavior.

The range indicator is always on the piece held in the hand, never on placed elements, to show where the other end will end up before placement. For placed parts there are other mods that cover that use case such as https://mods.factorio.com/mod/UndergroundIndicators which you might want to try instead.