"allowedFacewear" in a heagear's config is ignored when using BIS_fnc_unitHeadgear in order to randomize heagears for custom units as explained there :
https://community.bistudio.com/wiki/Arma_3_Characters_And_Gear_Encoding_Guide#FIA_headgear_and_facewear_randomization
{F27236}
Use the repro addon provided.
It adds a "REPRO soldier" under "OPFOR > Men".
Just spawn it an watch the result :
http://imgur.com/viY1m1d
A repro addon is provided. Config can be seen on the BIF :
https://forums.bistudio.com/topic/185036-questions-regarding-bis-fnc-unitheadgear/?p=2921643