Resolved
A distance filter in a View works directly from the Maps cache of addresses. If posts with an address field are imported they will not be present in the cache, and so will be missing from a View that includes a distance filter.
A button “Check for missing cache entries” has been added on the page Toolset > Settings > Maps.
After importing content, this should be used to run a batch process that will ensure all address field values have corresponding entries in the cache.