Hi Ryan.
The CSV export functionality in Subsurface is hard-coded to only
export the profile data for the first dive computer in any dive.
So for you to get what you want is to rearrange the order of dive
computers in your log. You can do this by selecting the dive
computer you want to export from (i.e. the Garmin Descent),
right-clicking on the dive computer name in the profile, and then
selecting 'Make first dive computer':
After this the exported CSV will be from the Garmin Descent, and include any heart rate information that is present.
Ngā mihi
Michael Keller