Not sure if maybe its not supposed to hit a target that has already been hit, but every other propagation spell that i've tested can bounce back and hit a target a 2nd or 3rd time if needed to continue finish off it's bounces.
Current character has 10 propagation and lightning bolt has taken a path in an L (3 targets) and with no continuable line, ends there (even if there were more targets in a different direction. Tested with weaken on the same target clump, and it hit said single on the L and just followed the path back to hit more targets (making a couple targets get hit twice).
#Propagation on Lightning Bolt not bouncing back
1 messages · Page 1 of 1 (latest)
You should also send your Logs on this
But 1: Do you have Diagonal propagation?
2: did an enemy die in the middle of the propagation?
I do think it is reason two. 
But just in case : You can find saves here :
%Appdata%/../LocalLow/Ishtar Games/The Last Spell/Save/Steam
(inside the numbered folder, it's your SteamID, then make an archive of the Profile1 folder)
And your logs here (whole folder):
%Appdata%/../LocalLow/Ishtar Games/The Last Spell/Logs