While there is no reason to prohibit users from doing so absolutely, it is recommended that they should not charge devices from the power station while it is charging. There are two reasons. Firstly, you might have an impulse to work on the device while it is charging from the power station. You will fall on the path of electricity in case something goes wrong and there is an accident. Secondly, you will not be able to use the power station to its full potential.
If you charge a device and draw out the power that is being gained by the power station, then the net charge gained by the latter becomes very low. The power bank battery will not end up charging properly because the device will take up all the power. You should not expect your power bank battery to charge and discharge at the same time.