Ah, so you’re talking about single leg FOCUS drills, rather than isolated leg training (ILT). My mistake. My best guess on the limited information available is still rooted in the execution of those drills. Single leg focus is not intended to shift all effort to one leg or the other, it is simply to focus on the pedal stroke of that one leg entirely. This might lead to an emphasized effort on one side, but you should not be sustaining 90% or 80% of your effort at target power with just that one leg. If that’s how you’re executing those drills, then the effect on your leg is similar to conducting ILT at target power in erg mode, and that’s still probably why you’re blowing up.
If that’s not how you’re doing it, then it could be as simple as your FTP is set too high for your current sustainable training capability, but it’s hard to say.