Similar here, as I get fatigued my HR will be a bit out of whack with the power (higher than expected for a given zone) which is clearly a result of fatigue. At that stage each workout of a given TSS actually provides a greater level of stress which accumulates and makes the issue worse… But then I think as long as I can manage to complete the workouts then that’s part of the process - perhaps lowering power to work from HR zones negates much of the training benefit. I’m not capable of answering that unfortunately though.