I’d like to suggest that the display when in Custom mode has an indicator to show if the period being used is the Current or Previous - perhaps a letter before/after the time period name?
At present to find this out you have to click on the measure and change to the period tab, for each one. As per the below image, it is unclear from looking at these which of current or previous these periods relate to - even though one is Current and the other Previous.
To get around this I am only using Current period and creating a separate time period as per below but would rather only have to manage a single time period:
