I know what you mean. I should have used the present perfect for the sentence, he did/does not change his mind toward her even in a situation, say, where Mayumi dumped A before.
But the first question was the reason why you can use the past sentence in the present situation. If I used the present perfect, I would have to explain the very complex relationship between the tense and the aspect, or even the mood in J grammer. So I simply used "did/does" in the sentence.
a)まゆみへの思いは変わらなかった。simple past
b)まゆみへの思いは変わっていなかった。past perfect
There are the gramatical differences between the aboves. And if you read only the sentence, a) or b), you think Mayumi should ask a question like
What does he think of Mayumi NOW???
And the sentence would be better that まゆみへの思いは(今でも)変わっていない。present perfect
Even thogh the differences, it does not matter if you use a) or b) in the situation, I suppose. Why? Because they love each others now and A just wants to show his love now using his past as an example.
There might be only a sentence "I love you", but there might be millions of ways to show your love to your partner.
英語の勉強を始めて10年以上経っても、私の英語力は変わっていなかった。