The only way to "fix" it, unfortunately, is to wait a little bit.

What's happening is that, when you switch targets on your main, that information has to go to the server, and then back down to all of your slaves. When we play all of these accounts together, we tend to think they should react together. But when the server gets involved, there's lag there, and there's no solution other than modifying your play style to accommodate.

The flame shock loss is important, obviously, so what I usually do is switch targets, cast LB or chain lit, then FS->LB.