Well, I/m having exact same issue, and here’s my conclusion after many tests.
With voidweaver talent, if you cast penance on a friendly target, no issue at all.
If you cast on a enemy target and changed to a friendly target when channelling, you lost bolts, no healing, no damage, no animation
If you cast on a enemy target and cancelled the target, same issue as above.
If you cast on a enemy target and switched to another enemy target, you will see 2 bolts flying out at the same time, but only the original target got the damage, the 2nd target only got fake animation.
If you cast on a enemy target and switched to a friendly target/cancel target, then change back again, you lost the bolts for the duration of not having a enemy target. just like 1 bolt at the begining and 1 bolt at the end.
So the conclusion is KEEP YOUR TARGET NOT CHANGED WHEN CHANNELLING. lol