It seems likely that this patchnote is what broke the behavior, assuming it was working properly prior to v1.66 like OP says:

This affects End Game Spectator and its derivatives. When spectating any non-local unit in first person, if their NVGs are on, you will not see any NVG effect -- mostly. Every ~18.5 seconds, you'll see a flash of the NVG overlay and green filter, but only for a few frames. Then it goes away again. In dark environments, first person POV quickly becomes completely unusable in EGS and derivatives due to a lack of working NVG modes. They can't even be manually enabled using camUseNVG.

Can confirm, been using an in-house solution that does exactly that for about two years now. It does indeed not work for slingloaded vehicles that have a driver, for the reason you stated.

This is a BI issue.

Relevant BIForums posts:
https://forums.bistudio.com/forums/topic/207471-172142223-small-marker-text-fix/
https://forums.bistudio.com/forums/topic/207472-etelkanarrowmediumpro-font-family-broken/

This issue is caused by changes to fonts[] under the class EtelkaNarrowMediumPro in CfgFontFamilies. Here is a temporary config workaround:

In T124875#1605925, @nunya wrote:

Dude really? You need to find a better hosting site. 48min wait to download file. Throw it on armaholic

This is a download link to a PBO which fixes this issue for those who find it insufferable like I do.