There appears to be an unintended interaction between at least one of the Druid of the Claw Hero Talents and the regular Druid talent Well-Honed Instincts. As a result of the interaction, manually casting Frenzied Regeneration when under 50% health will cause the Well-Honed Instincts debuff to be applied at full duration (two minutes). Frenzied Regeneration in this case still incurs the full rage cost (10), and still consumes a charge.
Switching the multiple-choice talents seems to have no effect on whether this occurs or not. My suspicion is that Wildshape Mastery may be at fault, however I’m unable to test without that specific talent due to the inability to leave points un-spent.
Note that Frenzied Regeneration will still work as normal if the Druid falls below 40% health. Switching to the alternative Hero Talent tree (Elune’s Chosen or Wildstalker, depending on specialization) also removes the errant behaviour.