There is not an issue with it failing to save them - it is the fact they are two different settings and one of them appears in two places that is confusing:
The units for rainfall can be toggled by clicking the scale on the rain layer, and your choice is also shown / can be changed in the settings panel.
The units for rain accumulation can be separately toggled by clicking the scale on the rain accumulation layer, but this setting is not shown in the settings panel
Either it should be consistent and show both choices in the settings panel, or (and i think this is the better option), have a single setting affect both the rain and rain accumulation layers. What use case is there for someone who wants to see rainfall in mm and rain accumulation in inches?