Hi, could you check the property that you are using to flag is not set as transient property. If you open the property from designer studio and go to "Advanced" tab check under "Persistence" section if "Do not save property data" is not checked. If it is checked the data will not persist.
FinishAssignment on submit button by default commit the case data, due to validation error commit won't happen in your case so when you refresh the section temp data will fade out. If you have save button in your case just do the save before the submit and check the flag you see the difference.