BUG #17944: Partial index on boolean field is not picked when using = while the index is created with is

The following bug has been logged on the website:

Bug reference:      17944
Logged by:          Ajeesh Sasidharan
Email address:      ajeesh2705@gmail.com
PostgreSQL version: 13.8
Operating system:   Linux
Description:

I have recently created a partial index on tables like following 

create index unprocessed_order_index
    on order (creation_date) where processed is false;

When I am querying the table using processed = false like below, the index
is not used 

SELECT count(*) from order where processed = false and creation_date <
now()

but the index is used while querying using processed is false

SELECT count(*) from order where processed is false and creation_date <
now()

I was expecting to treat both = and is same, or am I wrong