I have had the same issue consistently for Collector, and now for Field Maps - running on both Iphone 11 and Ipad 5 mini. I have had issues with employees getting caught out in remote areas with no access to their maps. My workaround was to download map areas onto the devices. Even if Field Maps does randomly decide to log out, as it does, I have ignored the prompt and I am still able to at least access the maps (no more lost employees!). However, I have not tried to collect as we are mainly using maps for navigation and database viewing while in the field. We use another product -- Fulcrum (a Spatial Networks product) for actual data collection.
