Cannot nbackup a firebird 3.0 database in firebird 4.0 service with engine12 setup in Providers

[ccy]

In firebird 4.0, I configure firebird.conf to support both engine12 and engine13 plugins:

Providers = Remote,Engine13,Engine12,Loopback

The firebird service may access both Firebird 3.0 and 4.0 database.

Next, I try to nbackup a Firebird 4.0 employee database and it works:

fbsvcmgr service_mgr action_nbak dbname c:\4.0\employee.fdb nbk_file r:\fb40.nb0 nbk_level 0

Finally, I try to use nbackup a Firebird 3.0 employee database:

fbsvcmgr service_mgr action_nbak dbname c:\3.0\employee.fdb nbk_file r:\fb30.nb0 nbk_level 0

and it fail with message:

unsupported on-disk structure for file c:\3.0\employee.fdb; found 12.0, support 13.0