Machine Learning algorithm for predicting number of cases in pandemic
It will depend completely on your feature engineering so I can think that in this case your model is maybe only predicting the mean or median of your target.

Also, it might help try using other kinds of models since you are trying to predict the counts of an event on a determined period of time, so it might be useful to use Poisson models that are in experimental phase in sklearn, nonetheless, the documentation might help to understand how the model works