make_copy fails with legacy tables created with AddGeometryColumn

I have a legacy PostgreSQL now in version 9.6 (PostGIS 2.5).

This database have been upgraded from previous versions and the old tables where created using AddGeometryColumn, so a \dlooks like this, with the check constraints present.

         Column          │         Type         │ Collation │ Nullable │                      Default                       
═════════════════════════╪══════════════════════╪═══════════╪══════════╪════════════════════════════════════════════════════
gid                      │ integer              │           │ not null │ nextval('myschema.mytable_gid_seq'::regclass)
the_geom                 │ geometry             │           │          │ 
Check constraints:
    "enforce_dims_the_geom" CHECK (st_ndims(the_geom) = 2)
    "enforce_geotype_the_geom" CHECK (geometrytype(the_geom) = 'MULTIPOLYGON'::text OR the_geom IS NULL)
    "enforce_srid_the_geom" CHECK (st_srid(the_geom) = 25829)

In the copy process of mergin-db-sync an error is raised when processing these tables:

== starting mergin-db-sync daemon == version 1.1.1 ==
Logging in to Mergin...
Processing Mergin Maps project 'admin/myproject'
Connecting to the database...
Downloading latest Mergin Maps project admin/inventario to /tmp/dbsync/myproject
The base schema and the output GPKG do not exist yet, going to initialize them ...
Error: copy failed!
GEODIFF: NOTICE:  schema "mergin_myschema" does not exist, skipping
Error: Unknown geometry type: GEOMETRY

Traceback (most recent call last):
  File "dbsync_daemon.py", line 63, in <module>
    main()
  File "dbsync_daemon.py", line 35, in main
    dbsync.dbsync_init(mc, from_gpkg=False)
  File "/mergin-db-sync/dbsync.py", line 647, in dbsync_init
    init(conn, mc, from_gpkg)
  File "/mergin-db-sync/dbsync.py", line 615, in init
    _geodiff_make_copy(conn_cfg.driver, conn_cfg.conn_info, conn_cfg.modified,
  File "/mergin-db-sync/dbsync.py", line 131, in _geodiff_make_copy
    _run_geodiff([config.geodiff_exe, "copy", "--driver-1", src_driver, src_conn_info, "--driver-2", dst_driver, dst_conn_info, "--skip-tables", _tables_list_to_string(ignored_tables), src, dst])
  File "/mergin-db-sync/dbsync.py", line 75, in _run_geodiff
    raise DbSyncError("geodiff failed!\n" + str(cmd))
dbsync.DbSyncError: geodiff failed!

With new tables that does not have the check constraints it works correctly:

Column │         Type         │ Collation │ Nullable │                       Default                       
════════╪══════════════════════╪═══════════╪══════════╪═════════════════════════════════════════════════════
 id     │ integer              │           │ not null │ nextval('myschema.testtable_id_seq'::regclass)
 geom   │ geometry(Point,4326) │           │          │ 

Not sure if this is a "bug" expected to be solved. Close it if you think that it is out of scope but it will be awesome if you have any advice or maybe a warning in the documentation can be added.

The failure is surfaced in dbsync.py at init() and _geodiff_make_copy(), through the geodiff copy command. Reproduce it with a PostgreSQL 9.6/PostGIS 2.5 legacy table using AddGeometryColumn and compare it with the working schema. Done means the legacy schema copies successfully, or the limitation is documented with a clear warning.