Adding variables to Random Forest decreases performance Let's say you have 6 variables.

A Random-Forest regression using the first 5 variables has an R^2 of 0.1. Another regression using just the 6th variable yields R^2 of 0.3. All of the first 5 variables are uncorrelated with variable 6 (&lt;0.1 correlation in absolute value).

Why would a regression with all 6 variables have an R^2 of 0.31 or 0.29, i.e. adding first 5 variables to the 6th variable only negligibly increases performance by 0.01 or even decreases it?

Notice that all models are tuned via random search cross-validation tuning depth, number of features, splits and number of trees.
Even if your variables are linearly uncorrelated, they may still be non linearly correlated. Just using the correlation coefficient to say the data is unrelated is not enough for non linear regression.

Still, adding variables to a model may not improve the end result if the variables cannot explain anything new from the target data. This seems to be such a case.