The rules on overlays were less clear in the past, something about visibility from 200m in normal darkness. At some point in the last 5 years or so they made a change to the VIRM to add more clarity. So that is likely why you got away with it in the past. Sections of the VIRM highlighted in yellow represent a recent amendment (I think they stay yellow for 5 years).
https://vehicleinspection.nzta.govt.nz/virms/in-service-wof-and-cof/general/lighting/rearward-facing-position-lamps
The inconsistant application of the VIRM by crappy inspectors is super annoying and I deal with this on a daily basis.
Customer says: "Oh well Bruce at Lowquality Autos has been passing it like this for years"
The virm is very clear on most things if you actually take the time to read it. Many WOF inspectors are lazy and won't fail cars on difficult issues if they think is going to cause a point of contention with a customer because it reflects badly on their business. The problem is that it then makes an inspector who is applying the rules correctly look like they are being difficult when the fail a car.
Stories like "I passed with x mod" do not mean they are legal from a WOF standpoint this is inspector inconsistency. If you are ever unsure of the legality of something have a read through the relavent section of the VIRM, that is the only difinitive answer.
https://vehicleinspection.nzta.govt.nz/virms/in-service-wof-and-cof/general/lighting/reversing-lamps
Reverse lamps are not mandatory equipment so you could tint that area of the light or even remove the bulbs if you wanted to.
https://vehicleinspection.nzta.govt.nz/virms/in-service-wof-and-cof/general/lighting/direction-indicator-lamps
Direction indicator lamps are mandatory equipment so you cannot tint over that area for a WOF