Not something I've noticed much in my playthroughs, I've been very impressed with the AI Signings during my current save, especially in the Prem with teams snapping up wonderkids for bargains/activating release clauses.
Also seen some nice expensive moves, like Liverpool buying 19 year old Lamine Yamal for £110 Million, which was cool to see.
The AI (like real life clubs) will make mistakes and buy players that don't work out. But if you have specific examples, we will need save files from before & after the transfers to review further.