When starting a two player career mode on PS5, my friend and I both selected F2 drivers (Paul Aron and Zane Maloney). It appears both of our driver numbers were set to 99.
We instead started another save with one F2 driver (Enzo Fittipaldi) and an F3 driver (Matias Zagazeta) icon instead, we noticed Matias had another number assigned other than 99, whilst Enzo still had the number 99 assigned.
It seems like F2 drivers in Two Player Career always get given the number 99 for some reason.