Function not found while creating partial index

Hello! The following series of statements results in an unexpected (to me)
error on 18.0 and 18.1:

CREATE FUNCTION my_function_1(n integer) RETURNS integer AS 'SELECT n'
LANGUAGE SQL IMMUTABLE;
CREATE FUNCTION my_function_2(n integer) RETURNS integer AS 'SELECT
my_function_1(n)' LANGUAGE SQL IMMUTABLE;
CREATE SCHEMA my_schema;
CREATE TABLE my_schema.my_table (my_number integer);
CREATE INDEX my_index ON my_schema.my_table (my_number) WHERE
my_function_2(my_number) = 4;

The CREATE INDEX fails with the following error:

ERROR: function my_function_1(integer) does not exist
LINE 1: SELECT my_function_1(n)
^
HINT: No function matches the given name and argument types. You might
need to add explicit type casts.
QUERY: SELECT my_function_1(n)
CONTEXT: SQL function "my_function_2" during inlining

SQL state: 42883

If I change the definition of my_function_2 to 'SELECT
public.my_function_1(n)' then the index builds successfully, so that's my
current workaround.

Thank you!