Engine swapping and upgrading bug problem

Product: Need for Speed: Unbound
Platform:PC
Summarize your bug This has been occurring since the vol. 3 update when I got the SRT Viper as a reward car by reaching level 50. As soon as I upgraded this reward car to max performance, it says I cannot upgrade it further and will remain at A+ class. This goes the same when upgrading my cars' performance level, or trying to engine swap it, the bug persists but it runs the standard speed when upgraded, just the cars' level that's being downgraded.
How often does the bug occur? Every time (100%)
Steps: How can we find the bug ourselves? Try to engine swap some of the other cars
What happens when the bug occurs? It downgrades the class level of my car down to it's lowest, from S+ class being pulled to A+ or B. But the speed remains the same
What do you expect to see? I hope this bug will be addressed because it really bothers me all the time into upgrading my car's class level

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I need help fixing this bug or error that's been happening to my game since the vol. 3 update. Please help me fix this issue so that I can customize my car's performance without any errors .