Does RFC-equipped Jayce fail to transform?

I was playing League of Legends' new game mode Team Fight Tactics with a friend the other day.

He had attached a Rapid Fire Cannon to Jayce, which improves his attack speed and doubles his range. At one point though, my friend began complaining that Jayce had failed to transform at full mana, and that he lost the round because of that.

When Jayce transforms, he first knocks away adjacent enemies, and then his attacks become ranged and gain a temporary but dramatic boost to attack speed. My suspicion though is that for this transformation to occur, Jayce must be adjacent to at least one enemy. I also suspect that a Jayce with the range to attack from a distance will not attempt to become adjacent and activate this spell.

Is my suspicion correct? Will an RFC-equipped Jayce fail to transform if his enemies do not enter an adjacent hex, or did my friend just experience unusual behavior from this character?

Yes, giving Jayce an RFC causes him to be unable to transform. This is because part of his transformation "spell", he knocks an adjacent enemy away. If there's no adjacent enemy, he can't cast the spell, thus he never transforms.

This is going to be fixed in the upcoming 9.17 patch - he will transform without knocking people away once he has enough mana.

Source: 9.17 Patch Notes