The 2019 BCA flag booklet have been reported with what is being referred to as "white/blue white" versus "green" tagging. These exist in various combinations of tagging on the 12-side versus 8-label side of the booklet.
While there is some discussion that the white may be a different type of taggant, I believe the same taggant is used in all cases. Two factors contribute to the effect people see. The images below explain these effects/differences and why I believe the same taggant is used on both.
This first image is of two booklets, one with the white and other with green tagging shown under SW UV.
The next image is of the same two under LW UV. Under LW UV the booklet with the white tagging (left booklet) is printed on a very hi-bright paper. Much more so than the green version.
This image is of the white version illuminated with SW UV under an optical microscope. The tagging grains are very sparse on this version.
The last image shows the green version under the same conditions above. The concentration of taggant is much higher/denser than the white version.
The combination of a very hi-bright paper and sparse tagging is leading to the perceived white tagging people see.