Solved

Staff Stars

You see 64% because you're getting a boost as your level is only 12. My level 20 teams get 40% of research power.

Do you know what level your old 4.5 star staff was? Because at some levels staff show different star ratings even though they are the same level and that already lead to, often frustrating, situations like this.

It's a problem with the stars rating. With your team being level 12 no level 12 staff should be 4.5 stars in the first place, they all should be 5 stars and as far as I know are treated as such by the game. Unfortunately sometimes resulting in situations like yours and I hope the devs get around taking a look at it.

I had the opposite case with another team. My team leveled but the stars rating of 4.5 of the staff did not change, but still the new car came out worse. Replacing the CD and TD with higher level but still 4.5 stars, all staff of my level I could find was only 4.5 stars, did improve the performance back to 5 star level.

Gustavo is right, the higher the team levels the lower the maximum research power gets. It's meant to equalize the teams a little bit and higher level teams should build their design HQ and thus are already closer to the max design points and in general should by then know how not to get too far behind in car design anyway. But still higher level staff is getting better results. It's just that the stars are sometimes not showing the proper rating. So it's better to judge staff by their level.