So I am one of those people who buys games and then when I am done I trade them in. 

What I usually do is check what trade in price I would get by GAME and if it's worth it, and then make a decision. The past 3 trade ins I have made, I've lost money compared to what the app showed and this is partially my mistake but this is where it gets weird.

GAME doesn't show trade in prices for the PS5 version of the game only the PS4 version. You would assume that PS5 versions being more expensive would get you more trade or at the very least the same amount, but they don't.

Take yesterday for an example, Miles Morales PS4 trade in price is £29. Because I traded in a PS5 copy, I got offered £22. 

GAME hasn't responded on Twitter and emailing their CEO has got me no reply. 

It just feels off and bizarre and feels like an underhanded tactic. 

Anyone else notice that with GAME or GS for our American counterparts?

My mistake was assuming the ps5 price on the trade in section of the app would be the same as the PS4 price or more since the games cost more to buy. Turns out they offer less for ps5 games 

The shop assistant was very honest said GAME don't update their prices often and it is out of their hands. 

That's not a GAME thing, but just how it works. I trade my games in a CEX as I don't have a GAME near where I live (and prices are usually always better there anyway).
Currently, Spiderman: Miles Morales is being bought for £20 on the PS5 and £28 on the PS4.

This is more simply supply and demand, with the pricing showing that they are getting more copies of the game traded-in on the PS5 than the PS4. This is true of any game - for instance you can often see disparities between the same game on Xbox and Playstation, just as with different console generations.
The best example is on the older FIFA games. Take FIFA 18. On Xbox One and PS4, they will buy it for only 10p. But the Xbox 360 version is being bought for £10 (PS3 £8) - the lesser legacy version running on out-dated tech but much harder to come by, so the rarity keeps the price high.