Goodmorning,
That is not a bug, but you probably applied the field the wrong way. The field key is determined by what the field in named in the form you use to have people register or add a listing. So phone works, telephone works and if you wish even Big Pink Elephant works. So the key can have any name you want.
If you want a phone numbr to display on GD Single template you can best use a badge. So add a badge where you want it to be, field key is phone (or telephone or whatever you use), field condition is not empty, and where it says 'badge' you enter %%input%%. This %%input%% makes sure the number is shown. If you want to display something like "call us at 2568948" you enter 'call us at %%input%%'.
Below that in clickaction you can use tel:%%input%% to make the badge clickable (very useful) so people can call in one click.
