There are many half cadence related threads in garmin forums. One example forums.garmin.com/.../incorrect-cadence
So I will say it is a classic bug. Reset has been tried. But nothing helps.
My device info
FR965 ASIA
Firmware 3.58
CIQ: 4.2.0
Fit files for you review https://drive.google.com/file/d/1kQvZkLetCyMTY3JyOEpZfhI_T-5d3IGE/view?usp=share_link
You can decode the fit files at https://www.fitfileviewer.com/
Due to half cadence. The stride becomes larger than usual.