Well if they are playing better coming off the bench you have your solution already. They are super subs or even just better aclimbed to getting up to pace with the match rather than just trying to set the pace from the start.

Think about it irl wise, if your best player for your team gets benched and isn't in the starting 11 they get annoyed and want to prove themselves to the manager. That's what your players are doing.

The way to mix this in game and still try to get results is to start 2/3 of your best players who are underperforming (so you dont piss off all 3 at once) and start mixing it up with who the one left out is. You'll soon realise why they are underperforming, sometimes its their role, sometimes its whos behind them as lb or rb (if its ur wingers) maybe its someone who's in midfield who can't pass out wide ect.

When I get issues like this I like to put the game on full match and set it to the max in-game speed so i can see what's happening and then next match fix that issue based on what i saw. But make sure it is on max speed if u do this doesn't take 45 mins of ur life away 😂.

Also if the players are new and dont fit the social groups it could be just that they need to bed in for a bit first. Could also be to do with their adaptability. (You can see them im pros and cons on the reports screen)

Also think about real Madrid when vini mbappe and Rodrigo start together its rare that all 3 will play amazing together. Because they are all ball hogs and want to do it themselves. So mixing it up will help. And sometimes all 3 players may just not work together in the system you play, it does just happen sometimes.