Enemies who currently possess the “Defiant” buff will display the “Interrupt!” float-text if they are hit with an appropriate CC ability while activating an ability, but will not be interrupted.
If “Defiant” enemies are intended to be completely uninterruptable, this is a display bug that misleads players into trying to interrupt them when timing is in fact utterly meaningless.
If “Defiant” enemies are intended to be interruptable but not CC-able, then the bug is that the ability casts are not interrupted.