Vacuum Full does not release the disk size space after delete from table

Hi hackers,

We meet a corner case that related to the behavior of Vacuum Full.

There are two SQL scripts

SQL1:
-------------------------------------------------------------
-- Test vacuum full

create schema s2;

set search_path to s2;

create table a (i int);

create table b (i int);

insert into a select generate_series(1,50000);

delete from a where i > 10;

-- Disk space should release after vacuum full

vacuum full a;

drop table a, b;

reset search_path;

drop schema s2;
-----------------------------------------------------------------------------------------

SQL2:
------------------------------------------------------------------------------------------
create schema s1;

set search_path to s1;

create table c(i int);

insert into c select generate_series(1,50000);

select pg_sleep(5);

reset search_path;

drop schema s1;
-----------------------------------------------------------------------------------------------

If we run both sql scripts on same database in parallel, the "VACUUM FULL
a;" will not release the disk space.

But if we run then on same database in sequence, "VACUUM FULL a" can
release the disk space.

We use the PG master branch and the autovacuum has set to off.

Could someone please help us to figure out the root cause of this issue?

Thanks a lot!

--
Regards,
Haozhou