[target=focustarget] not working for Mind Flay

I'm dualboxing a pair of priests (single system, keyclone, two WoW windows running on seperate monitors, macaroon)

In general the secondary character focuses the leader and all my macros are like this:

Character 1:
/cast Smite

Character 2:
/cast [target=focustarget] Smite


This works as expected for everything except Mind Flay. When /cast [target=focustarget] Mind Flay is used I normally get a "You do not have a target" error but very rarely it will work and once it mindflayed a completely different target. This happens even if the lead character is standing there, not casting, with a mob targetted; all other macros work but mind flay fails. The button does correctly change colour based on range to the focus target.

I've worked around this by using "/target focustarget /cast Mind Flay", but I'm wondering why I had this issue in the firstplace; is this some sort of odd behavior caused by Mind Flay being a channeled spell?