My players value is ridiculously low

So I've been trying to figure out why my own players are worth so little compared to any other player. I was thinking of selling my left back Juan Sanchez Mino for around 12 million euros, despite his value being roughly 4 euros. But if you would compare Juan Sanchez Mino to any other left back within the price range of around 10+ millions, the stats would be pretty much the same. What I'm saying is that the valuation of my player is terribly wrong. But anyways, I tried selling him for 5 million (since that's the highest price anyone would offer me) and voilà. When the transfer was a done deal, his value had now increased to 10 million euros. From having a value of 4 millions in Catania, to 10 millions in Atletico de Madrid, in just one transfer.

Is anyone having the same problem? Because it's starting to piss me off.

Grateful for any response!