Confused AUC ROC score
Oh, I think I've finally got it.  It's just an averaging problem: for each fold in your k-fold cross-validation, you get perfect auROC, but at the default threshold of 0.5 your hard classifiers (for each fold) sometimes have $FPR=0$ and $TPR&lt;1$, but some other times $FPR&gt;0$ and $TPR=1$.  Then averaging you are able to get both $\operatorname{mean}(FPR)&gt;0$ and $\operatorname{mean}(TPR)&lt;1$.
To check, have a look at the cv_results_ table, particularly at each test fold scores (split&lt;i&gt;_test_&lt;xyz&gt;), rather than just the mean_test_&lt;xyz&gt; scores.