MMH5.5 - Heroes of Might and Magic 5.5

I'm creating a map using MMH55_Utility_64.
Map size: 149KB
(the map is working)

I open the editor and make changes.
When saving:

The map size becomes: 1KB
(the map is broken)

You don't even need to do anything in the editor. Just load the map and close the editor. The map still breaks.

P.S. Maps not created with MMH55_Utility_64 can be edited normally.