BUG #17407: trim trims more than expected

The following bug has been logged on the website:

Bug reference: 17407
Logged by: trim combined with substring from pattern for gives inconsistent output
Email address: unmesh(dot)joshi126(at)gmail(dot)com
PostgreSQL version: 12.4
Operating system: Linux 5.15.4-051504-generic
Description:

Expected output

demo=# select trim ('nextval(''' from
substring('nextval(''database_table_id_seq''::regclass)' from
'%#"%_id_seq#"%' for '#' ));
btrim
-----------------------
database_table_id_seq
(1 row)

Unexpected output

demo=# select trim ('nextval(''' from
substring('nextval(''auth_table_id_seq''::regclass)' from '%#"%_id_seq#"%'
for '#' ));
btrim
------------------
uth_table_id_seq
(1 row)