Why delete and insert in a CTE works despite UNIQUE constraint?

Given a table with a unique constraint on a column

CREATE TABLE t(    id   int GENERATED ALWAYS AS IDENTITY,    name text NOT NULL UNIQUE);

Let's create a single record

INSERT INTO t (name) VALUES ('a');

When I try to delete an existing record and insert a new one with the same value for the unique column in a single statement using CTE, it fails as expected by me:

WITH    deleted_cte AS (DELETE FROM t WHERE name = 'a' RETURNING id),    inserted_cte AS (INSERT INTO t (name) VALUES ('a') RETURNING id)SELECT 1;-- ERROR:  duplicate key value violates unique constraint "t_name_key"-- DETAIL:  Key (name)=(a) already exists.

I expect DELETE and INSERT commands to run concurrently in an unspecified order and see the same snapshot of the table.

However, if I introduce dependency between the primary query and the deleting sub-statement, it works:

WITH    deleted_cte AS (DELETE FROM t WHERE name = 'a' RETURNING id),    inserted_cte AS (INSERT INTO t (name) VALUES ('a') RETURNING id)SELECT id from deleted_cte; -- only this line modified-- 1 <- returns id of the deleted record SELECT id FROM t;-- 2 <- inserted record-- Plan of the query:-- CTE Scan on deleted_cte--   CTE deleted_cte--     ->  Delete on t--           ->  Seq Scan on t--                 Filter: (name = 'a'::text)--   CTE inserted_cte--     ->  Insert on t t_1--           ->  Result

I do not understand what happens here.

Are sub-statements ordered now and INSERT is forced to run after DELETE?
Is UNIQUE check somehow moved to the end of the whole CTE?

Where in the docs can I read about this behavior or is it an unreliable thing?

https://dbfiddle.uk/FYrW2n5W

I realize that this exact delete-insert can be replaced with upsert.

But in the actual code records aren't hard deleted. They instead are being soft deleted using UPDATE t SET deleted_at = now() and then new records inserted. Unique constraint filters out soft deleted records CREATE UNIQUE INDEX ON t (name) WHERE (deleted_at IS NULL). So upsert wouldn't work here.