In Windows, if there are two or more user accounts, Steam only works correctly for the first user who launches it.
The problem occurs as follows:
1. User A logs into Windows and opens Steam → everything works.
2. User B logs into Windows (without logging out User A) and opens Steam.
3. At that moment, Steam automatically closes in User A's session and opens only in User B's session.

In other words, it is not possible to have two instances of the Steam client open simultaneously in different Windows sessions, which interrupts the first user's use.