BUG #16486: Prompted password is ignored when password specified in connection string

The following bug has been logged on the website:

Bug reference: 16486
Logged by: Alexander Lakhin
Email address: exclusion(at)gmail(dot)com
PostgreSQL version: 13beta1
Operating system: Ubuntu 18.04
Description:

Assuming that we have the user u1 with the password p1, the following
command:
psql -U u1 'dbname=postgres password=p1' -W
prompts for a password but the input password is effectively ignored.
So if the connection string contains an invalid password:
psql -U u1 'dbname=postgres password=p2' -W
you can enter a valid password in a prompt, but still could not connect.
The documentation says:
PGPASSWORD behaves the same as the password connection parameter.

But in fact PGPASSWORD behaves differently . The following command:
PGPASSWORD="p1" psql -U u1 'dbname=postgres' -W
prompts for a password and fails if the password is wrong.
Similarly, if the environment variable contains an invalid password, it
still can be overridden with "-W".