problem about reindex

I'm using PostgreSQL 12.7 on RHEL 8.4. I have a table that has 3132 rows. I faced an interesting case when I run this query.

select * from enterprise.private_room where name = 'mannheis';

 id | name | user_id | create_time | sub_package_id | password | tenant_id
----+------+---------+-------------+----------------+----------+-----------
(0 rows)

But the table has this row:

select * from enterprise.private_room where lower(name) = 'mannheis';

 id   |        name         |               user_id                |        create_time         | sub_package_id | password | tenant_id
-------+---------------------+--------------------------------------+----------------------------+----------------+----------+-----------
 67182 | mannheis             | f4e14cbe-f8e2-4c04-85cf-30271cc08526 | 2020-06-15 14:35:51     |         101361 |          |         3

I got the same result with this query : select * from enterprise.private_room where trim(name) = 'mannheis';

After doing reindex the table, the problem was solved. But I don't understand why this problem occurs.

Thanks in advance,

Liam