I have field teams that frequently need to take map areas offline while using Field Maps to collect data. I have found that once a map is set up to be taken offline, the hosted feature layers within that are editable are converted to WGS84--meaning, after the collected field data is synced, and then exported to a File Geodatabase, the spatial reference for this data will be WGS84.
To be clear, these layers were originally published in our State Plane coordinate system (except the basemap, that's WGS84), and I do not have this same experience when using live syncing (NOT taking the maps offline). In cases where cell service is decent, and we don't need to set up maps as offline capable, the exported feature layers seems to maintain the original State Plane coordinate system that they started with when published.
Can anyone confirm that this is expected behavior, and/or if there is a workaround? My guess as to what is happening is that an offline device cannot manage complex coordinate system transformations on the fly, so everything is transformed into Web Mercator once a map is "offline enabled."