At what point in the season did this occur?
It sounds like the offseason - Value is tied to Reputation/position/recent performance,
If no major reputable international competition occurred over the offseason, and your league's reputation did not rise due to performance of other clubs in it/no major sales of players whose value rose significantly upon leaving/performed well since leaving then these may be why the values have diminished
League reputation is also tied almost exclusively to European coefficients. Not the seasonal one either but the 5 yearly one - obviously not instantly but eventual creep.
If none of the above then I'm out of ideas!