Hello,
With DDPG, a common thing to do in the final 3 layers of the actor is to use a fully connected layer, a tanh layer and a scaling layer. Tanh will get the ouput of that layer between -1 and 1 and then you can use the scaling layer to scale/shift values as needed based on the specifications of the actuator in your problem.
It seems the problem here is due to noise that is being added during training with DDPG to allow sufficient exploration (for example see step 1 here). The default noise options have a pretty high variance, so when this is added to the output of the tanh layer, it ends up outside the [0, 1] range and is being clipped. This is why you are only getting the two extremes.
Try adjusting the DDPG noise options, and particularly the variance (make it smaller, e.g. <=0.1). Also, see here for some best practices when choosing noise parameters.
Hope that helps
