When select an opacity from standard list (10%, 20%....) then next time one consults same menu for same part that opacity should show with a Tick next to it.
Relatedly, if a custom opacity has been previously selected then there should be a Tick (or some other visual change) next to the "Custom..." option in the dropdown.
As per this thread:
https://forums.autodesk.com/t5/fusion-360-support/selected-opacity-does-not-remain-checked-in-dropdo...