Isn't only tied to the performance of his club Napoli alone, but also tied to his individual performances in both the country and in the club side as well, because during the African Cup of nation , many fans and supporters believed/expected Osimhen to have done incredibly well in that competition and help Nigeria in lifting the trophy, but he performed way far below expectations and couldn't help Nigeria out in the nation's cup competition, I believe that helped drop his rating and market value.
Osimhen did performed well at the AFCON, but he wasn't have a perfect blend for team mates which will mean he may not be able to perform beyond the teams capacity, a typical example can be seen with Ronaldo who alone couldn't help Portugal get to win world cup of even having a very spectacular and brilliant performance in the Euro 2024, a single player cannot get his team delivered all the time if not provided with the necessary help he needs to perform optimally. In Napoli his first season was very awesome, then he got all the support but in the most recent season, there were issues surrounding his presence in that team and the need to leave became paramount which may have been a very paramount factor to why his performance dropped.