If a user is filling out a form that uses connectors, these will time out after a period of time (connecting the people picker field to an Azure Active Directory group, for example). If a user has a form open on their computer and the connector times out, the user cannot load any data from the people picker.
My suggestion is for open forms to auto-save and "log out" the same way the automation designer logs you out after a period of inactivity. A user can simply log back into their My Nintex and go back into the saved form to continue it if need be.