On topic, this is just a bug that needs fixing, but I would like instead to talk off topic on witch bolt balance.
Witch bolt it incredibly underpowered. Its pretty hard/lucky to use it for 2-3 turns without losing concentration, or having the target get out of range etc, and in case everything works right, the numbers give no meaningful reward.
But, its not at all bad design for a game to have inefficient spells. It makes the choice of spells feel more meaningful, allows bad guys to have interesting but not too good spells to hit the party with, and also presents the potential of adding hidden interesting items that buff it so much it becomes good.
A few suggestions for such items, with different strengths:
1) Feedback/Overloaded Witchbolt - The spell retains the energy from the previous turn and the
channeled extra energy is added to it: first turn 1d12, 2nd turn 2d12, 3rd turn and above 3d12 but
the caster also takes 1d12 damage and has to roll a concentration check to keep the spell for the
next round.
[variation: you only roll 1d12 each turn, and add the damage of the previous turn, up to a max of 36/turn.
any damage above 24/turn is also inflicted on the caster (i.e. at 30/turn, caster takes 6hp of damage)]
2) Redirected Witchbolt - A bonus action is enough to maintain the spell, allowing the caster
to cast other spells in subsequent turns in addition to maintaining the spell.
It can be cast on two enemies, connecting them to each other with the bolt
until they get out of range of each other. They will both take damage.
3) Leeching Witchbolt - heals the caster for half the damage dealt.
4) Chaining Witchbolt - The target cannot move outside the range. The range can be reduced by the caster
at will to a minimum of 15 feet. (can be overcome with str check, taking another 1d12 damage upon breaking
the spell)
5) Forking Witchbolt - 3 extra bolts connect the target with other enemies, in addition to the main bolt.
The extra bolts have a range of only 15 feet, and deal 1d6 each turn.
6) Line witchbolt - enemies between you and your target are also damaged.
7) Shielding Witchbolt - the damage is converted to temporary hp on the caster lasting 1 turn.
