I just tried to delete my display name in the account settings and it didn't work. I deleted the name in the box and pressed the save button at the bottom. Then I returned to my profile and the display name was still there. Refreshing and reopening the app didn't change that. Returning to the account settings, the display name was back in the box. I tried several times with the same results. Changing the name does work, but removing it doesn't. I thought at first it might just not be possible in Lemmy, but I successfully removed it in Jerboa.