The feature/id command in the Beta version of the REST API retrieves all features that overlap the ID that you've put in, not just transcripts of that gene. If we look at the region of the gene, we can see that it overlaps five other transcripts (two genes) on the opposite strand. The feature/id command has been renamed the overlap/id command in the newer full release of the REST API.

At present, I'm afraid there is no command in the REST API to get all transcripts of a gene. You'll have to use the Perl API instead to get this kind of data. It would be something like:

use strict;
use warnings;
 
use Bio::EnsEMBL::Registry;
use Bio::EnsEMBL::SeqFeature;
my $reg = "Bio::EnsEMBL::Registry";
 
$reg->load_registry_from_db(
   -host => 'ensembldb.ensembl.org',
   -user => 'anonymous'
);

my $gene_adaptor = $reg->get_adaptor ('human', 'core', 'gene');
my $gene = $gene_adaptor->fetch_by_stable_id( 'ENSG00000133703' );
my @transcripts = @{ $gene->get_all_Transcripts };
while (my $transcript = shift @transcripts) {
# print some stuff
}