Low HR for Zone 2 and 3 Training

Hi Fellow Coaches
So currently working with a 45-year-old male athlete who is base training in zone 2 and zone 3. The Athletes HR zones are
Z1 0-124
Z2 125-138
Z3 139-144
Z4 145-154

The athlete continues to have no problems surpassing power targets. However, I cannot get the athlete’s HR zone up to match the power zones of the efforts. For example, I continuously have the athlete ride Z2 at 130 bpms and Z3 at 140+ bpm. However, I am finding this athlete riding at or above their 240 thresholds to achieve these numbers. The athlete cannot stay there for very long. I have been working with the athletes for several years, and this year I have noticed it dramatically header to get the athlete to increase HR zones. The athlete has had stress tests, echos, lab test and all have come back normal. I have ensured the right recovery and rest periods are built into the training and watch recovery scores carefully. I have reached out to our local performance lab to ask the questions with no real answer except for fatigue or age-related declines in HR. Has anyone else experienced this and what was your take on it. Is there any related research on this topic of declining HR in Z2 and Z3 work.