GridSearchCV Decrease performance RF
These are different for a valid reason
The standalone model is overfitting. You can see that with r2score of 84 Vs 22.
The reason for that is, the standalone model goes to full depth and hence overfit the train data and badly fit the test data.

max_depthint, default=None
The maximum depth of the tree. If None, then nodes are expanded until all leaves are pure or until all leaves contain less than min_samples_split samples.

While with CV, you have parm for max_depth. Hence bad but Truth.