I don't trust these charts much - especially the section where the price has risen sharply in a short time is questionable. He is not getting cheaper, but returning to normal (and he is still a very expensive player and his price reflects his value).
That surge was caused by the situation when Bayern was trying to get Kane, Real Mbappe, the Saudis were also nearby, plus there was not much choice on the market. The fight between several top clubs for a limited resource drove up prices on everything.
I respect your opinion on charts. Of course, it doesn't always show the price the team will sell the player for.

The time period from the day his market value started to drop down to this day can be explained like:
-
First huge problems broke out at Napoli and it reflected on his effort in time-
Later on he signed for a team from a much lower European league so naturally his market value dropped drasticallyIf he didn't have any problems at Napoli, probably he would still be valued over 100 million euros.
