OS upgrade vs Postgresql

Hi team,

Our set up is Site1(primary - rhel 9.6, postgres 17.6 in a patroni set up+
pgbackrest 2.6v & sync replica - - rhel 9.6, postgres 17.6 in a patroni
set up+ pgbackrest 2.6v) and in site2(async standby - - rhel 9.6, postgres
17.6 in a patroni set up+ pgbackrest 2.6v)

We have OS upgrade from rhel 9.6 to 9.7 for two nodes in Site1. We can see
postgres and pgbackrest has automatically upgraded to 17.7 and 2.57
respectively.

Whereas db node in site3 is no upgraded and stays with 9.6, 17.6 and 2.56
for rhel , postgres and pgbackrest version respectively.this will be
upgraded tomorrow

Is this a problem( we intend to upgrade only OS and not postgres) ? Is this
problem or is it best way or is it a normal behaviour that can or should be
avoided?
What wrong we are doing?
What is the right approach?
Any blogs is appreciated