When ecs compability is disabled, the default template that an Elasticsearch output may install will contain a mapping that makes that a geo_point.
Speculation: When ecs_compatibility is enabled then target is a required field unless source has one of a small set of values. As such, since it is unclear what the target of the filter is going to be, the ecs compatible templates (which I cannot find) may not have any fields be a geo_point by default.
Answer: you need to add a template or otherwise set the mapping on the index. Note that you cannot change the type of a field once created. You will need to create a new index.