The Bicycle reference
Its rule prevents leading a spade until one has been played earlier, unless the leader holds only spades. Distinguish leading a trick from playing a card later in that trick.
Try an open-hand explanation
During practice, identify who is leading and which cards that person can legally choose. Then explain the separate choices for the next player. Consult your selected version when the explanations disagree.
Confirm exceptions first
An app or house rule may use another lead restriction. Write its setting down before a scored hand rather than applying a remembered exception midway through play. This note concerns the normal deck reference, not a Joker variant.