Commented by: @hvlad

1. Prepare database

create database core-5143.fdb;

create table t1 (id int);
create table t2 (id int);

set term ^;
create function f1 returns int
as
begin
return (select count(*) from t1) + (select count(*) from t2);
end^
set term ;^

2. Make backup
gbak -b core-5143.fdb core-5143.fbk

3. Restore using switch -one_at_a_time
gbak -rep -v -one core-5143.fbk core-5143.1.fdb
gbak:opened file core-5143.fbk
...
gbak:restoring function F1
gbak: restoring argument for function F1
gbak:restoring table T1
gbak: restoring column ID
gbak:committing metadata for table T1
gbak:error committing metadata for table T1
gbak: ERROR:table T2 is not defined

!!!

gbak:restoring table T2
gbak: restoring column ID
gbak:committing metadata for table T2
gbak:error committing metadata for table T2
gbak: ERROR:unsuccessful metadata update
gbak: ERROR: TABLE T2
gbak: ERROR: Can't have relation with only computed fields or constraints
gbak:committing metadata
gbak: restoring privilege for user SYSDBA
gbak: ERROR:action cancelled by trigger (0) to preserve data integrity
gbak: ERROR: could not find object for GRANT
gbak:Exiting before completion due to errors

4. Restore NOT using switch -one_at_a_time
gbak -rep -v core-5143.fbk core-5143.1.fdb
gbak:opened file core-5143.fbk
...
gbak:restoring function F1
gbak: restoring argument for function F1
gbak:restoring table T1
gbak: restoring column ID
gbak:restoring table T2
gbak: restoring column ID
gbak:committing metadata
gbak: restoring privilege for user SYSDBA
...
gbak:creating indexes
gbak:committing metadata
gbak:fixing views dbkey length
gbak:updating ownership of packages, procedures and tables
gbak:adding missing privileges
gbak:fixing system generators
gbak:finishing, closing, and going home
gbak:adjusting the ONLINE and FORCED WRITES flags

Commented by: @dyemanov

I see that packages are affected the same way. Solution looks simple - just revert bugfix CORE949 because it's superseded by bugfix CORE374, which works equally well for procedures, functions and packages.