I, too, have been struggling with this mightily. Some of you may know that custom dynamic registrations are sort of my calling card as I utilize it in many, if not most, of my liveries. I keep looking for a way to fix this nagging issue.
I think I've narrowed it down to being a compatibility issue with 2020 aircraft in 2024. In short, I've had no problems with dynamic registrations in aircraft using the native 2024 folder structure, especially the modular format. Any 2020 aircraft or 2024 aircraft using the legacy (2020) folder structure either does not have working dynamic registration or is limited to just the default font and color (that is, black).
It might be that our hands are unfortunately tied with any 2020 aircraft being stuck with default dynamic registration and work around that or, again, we ask Asobo to look into this and hopefully fix it.