Battleye Arma 3 64bit blocks opentrack's NPClient64.dll - there is a Battleye message that this file has been blocked.
Tested with opentrack 2.3.10, 2.3.9 and 2.3.1.4
https://github.com/opentrack/opentrack/releases/tag/opentrack-2.3.10
This prevents any head tracking in 64bit Arma 3 that is based on opentrack.
32bit Arma 3 works fine (in that case file NPClient.dll is used) and tracking device is available.