It's a bug, not sure what is causing it exactly.
Wouldn't mind if it was fixed...

I've seen reports on this, from people using IS Boxer, Keyclone and one other software option.
So its hitting three different multiplexers..
Guessing its something to do with 3.3 and the addon API.

For me, it seems to hit if I've had strobe on.
Have switched to a new leader.
And enable strobe with a different leader then the first from the session.
And get into any kind of combat, with strobe on the new leader.
That leader seems to want to then follow the original strobe target.

Not using Strobe, or disabling it before combat is my fix so far.