I agree it is a bug and you should report it if it bothers you.
for bug report:
"Hotkey A - heal set to cursor target cursor
Hotkey B - heal set to target cursor
Hotkey C - heal set to cursor target self
Hotkey L = action cursor target last
Hotkey S = heal set to cursor target stored
T = some target mobile
set T to current target in target window, then A B L fails to cast 2nd spell on T. C B L also fails, it should target self again but does target T. If you use hotkey S with the stored target being T, S B L also fails on the last target cast. " or just report what you wrote in your op.
My preference is to have every spell target set up as cursor, tho I also have some duplicates for fast spells to either current target or target self.
The workaround if you want a single key current target spell is to make a macro like M below. if you have set current target T in your target window, key sequence M B L will target T for both spells. I think this is what you are trying to do.
Hotkey M = 3 part macro: heal target cursor, wait for cursor, cursor target last