problem about reindex

Hello team,
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