Orange 3 - Same prediction for all of my data when using impute widget
When imputing, certain characteristics of the data have to be recorded. E.g. when you impute missing data by mean value parameter $\mu$, you have to record that $\mu$ from training data in order to be able to reapply the exact same transformation on the test data.

Thus you only need to connect your training data to the Impute widget, while the testing data will be imputed with the same parameters when the two domains are matched (in Predictions).

If you connect testing data to its own Impute widget, it gets imputed with its own, testing-data-specific parameters, which is, as you have discovered, incorrect.

See also: Difference between fit and fit_transform in scikit_learn models?