It should be fixed in 1.0: https://github.com/graphhopper/graphhopper/pull/1917
If you still encounter problems please write a simple reproducer here and make sure you use the correct format (see the api docs)
Additionally I would recommend to use the customizable routing instead where you would also have the possibility to make the area restriction conditional (e.g. only for bigger roads in this area or similar) or only avoid an area or use it in the speed mode. And the block_area feature is planned to be deprecated (but currently is not).