The following bug has been logged on the website:

Bug reference: 17720
Logged by: reiner peterke
Email address: zedaardv(at)drizzle(dot)com
PostgreSQL version: 15.1
Operating system: openSUSE Leap 15.4
Description:

I have a table
create table hamster(under integer, over text);
I create a unique index on the under column with nulls not distinct
create unique index uq_not_distinct on hamster (under) nulls not distinct;
i now create a primary key using the unique index

alter table hamster add constraint pk_hamster primary key using index
uq_not_distinct;
NOTICE: ALTER TABLE / ADD CONSTRAINT USING INDEX will rename index
"uq_not_distinct" to "pk_hamster"
table looks good
\d hamster
Table "moon.hamster"
Column | Type | Collation | Nullable | Default
--------+---------+-----------+----------+---------
under | integer | | not null |
over | text | | |
Indexes:
"pk_hamster" PRIMARY KEY, btree (under) NULLS NOT DISTINCT

Do a pg_dump of the database.
the dump creates the code for a primary key that cannot be restored
pg_dump -p 5632 -Of tranquility.sql -d tranquility
on restore, I get the following error
psql:tranquility.sql:90: ERROR: syntax error at or near "NULLS"
LINE 2: ADD CONSTRAINT pk_hamster PRIMARY KEY NULLS NOT DISTINCT...
in the dump itself the create constraint command is
ALTER TABLE ONLY moon.hamster
ADD CONSTRAINT pk_hamster PRIMARY KEY NULLS NOT DISTINCT (under);
which does not work with the NULLS NOT DISTINCT string