Partial agonists have weak agonist activity on their own (thus in this case it causes HR to increase, b-adrenergic effect) but when an actual agonist is present (aka when you are exercising, you are producing NE and E that have full b-agonist effects), partial agonist actually have a mild antagonist effect (thus the heart rate decreases).

A key property of partial agonists is that they display both agonistic and antagonistic effects. In the presence of a full agonist , a partial agonist will act as an antagonist, competing with the full agonist for the same receptor and thereby reducing the ability of the full agonist to produce its maximum effect. Ex. Morphine (full agonist) vs buprenorphine (partial agonist) at opioid mu receptors 234 FA 19

Partial agonist: https://link.springer.com/referenceworkentry/10.1007%2F978-3-540-68706-1_548#:~:text=A%20key%20property%20of%20partial,to%20produce%20its%20maximum%20effect.

In the case of exercise, there is increased sympathetic tone (acts as full agonist).