The key to store and forward with MQTT and Sparkplug B is the use of the Primary Host ID/Topic. You set this on the subscriber that you want to be the most important. When the connection to that subscriber(via its primary host ID/Topic) is lost, the RIO/EPIC will go into store mode, when that connection is re-established it will backfill all historical data into that system.
You can set the Primary Host ID/Topic in native data services and Ignition:
As well as at the primary subscriber(Ignition in this instance):
As long as the name matches on all systems you are set.