currently non-discovered items are immediately marked as missing. if
ZBX-4475 is fixed, they would also be stopped being monitored.
in many cases that is ok, you do not want to poll snmp interfaces that went away - but in some cases an entity might disappear for a brief moment, and then monitoring of it would stop until next discovery run, which usually will be hours.
there should be a way to tell zabbix not to mark those items as missing - in the most simple case it could be a checkbox, but a better approach would be input field to say after how many failed discoveries items should stop working. for example, somebody could set it to 3 so only 3 consequent discoveries missing something would mark it for future deletion and stop monitoring it.
to emulate the current behaviour, it would be set to "1"