By default the Change Radius command is the first command in the mini toolbar when you select a single cylinder face. Automatically the copilot becomes visible and you can change the radius by dragging this arrow, without the need to first activate the command.
Since version 18.1 the possibility exists to modify the content of the command mini toolbars. If you change the Single Cylinder face MT and put the change diameter command on the first place, then you don't get the copilot automatically and you need to activate the command first before the copilot appears.
Both the commands are pretty similar so I hope this behavior will be unified.