Why is my function inlined only when STABLE?

Hi all,
I have a function that isn't being inlined, and I would appreciate help to understand why that's the case.

I'm using PG 11.15. I know that if I declare my function IMMUTABLE and it calls a non-IMMUTABLE function, Postgres won't inline my function. But even when my function calls only substring() (which I understand to be IMMUTABLE based on '\df+ substring'), I still can't get Postgres to inline it. If I re-declare my function as STABLE, then Postgres inlines it. According to the rules I understand (https://wiki.postgresql.org/wiki/Inlining_of_SQL_functions#Inlining_conditions_for_scalar_functions) the IMMUTABLE version of my function should be inlined too. What am I missing?

Here's a log of a CLI session showing that the IMMUTABLE version is not inlined, but the STABLE one is.

show track_functions
+-------------------+
| track_functions |
|-------------------|
| all |
+-------------------+
SHOW
me@/tmp:wylan#
SELECT * FROM pg_stat_user_functions
+----------+--------------+------------+---------+--------------+-------------+
| funcid | schemaname | funcname | calls | total_time | self_time |
|----------+--------------+------------+---------+--------------+-------------|
+----------+--------------+------------+---------+--------------+-------------+
SELECT 0
Time: 0.021s
me@/tmp:wylan#
CREATE OR REPLACE FUNCTION f(foo text)
RETURNS text
AS $$
SELECT substring(foo FROM 1 FOR 2)
$$ LANGUAGE sql IMMUTABLE PARALLEL SAFE;
CREATE FUNCTION
Time: 0.003s
me@/tmp:wylan#
select f('4242')
+-----+
| f |
|-----|
| 42 |
+-----+
SELECT 1
Time: 0.008s
me@/tmp:wylan#
SELECT * FROM pg_stat_user_functions
+----------+--------------+------------+---------+--------------+-------------+
| funcid | schemaname | funcname | calls | total_time | self_time |
|----------+--------------+------------+---------+--------------+-------------|
| 14472085 | public | f | 1 | 0.05 | 0.05 |
+----------+--------------+------------+---------+--------------+-------------+
SELECT 1
Time: 0.022s
me@/tmp:wylan#
DROP FUNCTION f(text)
DROP FUNCTION
Time: 0.001s
me@/tmp:wylan#
CREATE OR REPLACE FUNCTION f(foo text)
RETURNS text
AS $$
SELECT substring(foo FROM 1 FOR 2)
$$ LANGUAGE sql STABLE PARALLEL SAFE;
CREATE FUNCTION
Time: 0.003s
me@/tmp:wylan#
select pg_stat_reset()
+-----------------+
| pg_stat_reset |
|-----------------|
| |
+-----------------+
SELECT 1
Time: 0.008s
me@/tmp:wylan#
SELECT * FROM pg_stat_user_functions
+----------+--------------+------------+---------+--------------+-------------+
| funcid | schemaname | funcname | calls | total_time | self_time |
|----------+--------------+------------+---------+--------------+-------------|
+----------+--------------+------------+---------+--------------+-------------+
SELECT 0
Time: 0.022s
me@/tmp:wylan#
select f('4242')
+-----+
| f |
|-----|
| 42 |
+-----+
SELECT 1
Time: 0.008s
me@/tmp:wylan#
SELECT * FROM pg_stat_user_functions
+----------+--------------+------------+---------+--------------+-------------+
| funcid | schemaname | funcname | calls | total_time | self_time |
|----------+--------------+------------+---------+--------------+-------------|
+----------+--------------+------------+---------+--------------+-------------+
SELECT 0
Time: 0.019s
me@/tmp:wylan#

Thanks
Philip