Hello,
First, please note that channel collisions are completely normal and expected, especially when you use burst data. Burst data seizes the whole radio from other channels.
To minimize channel collisions, I suggest you ensure your HRM channel is set to low priority search. If the HRM search is left as the default high priority search, it will seize control of the entire radio from all other open channels when it is opened. This could be the reason why your successful burst rate drops so low when the HRM channel is opened.
I hope this helps,
Kassandra