There is new functionality that is intended to make it so skills and effects which have a
chance to grant an extra turn start suffering from diminishing returns (to reduce the chance of getting lucky with 25% chances for, say, 5 turns in a row). However, there appears to be a bug where skills with a
guaranteed extra turn also use this decaying chance. This is a bug we thought we had fixed before release (for situations like Warrior/Assassin Child of the Damned's Shadow passive), but it appears the fix did not also work for guaranteed extra turns from skills (such as Vampire Hunter Eddie's Mark).
You can see the rules and decay amounts for extra turn chance in the Character Tuning section of the
patch notes.