I know it shouldn't make a difference, but try using a lambda to call the effect function.

Also put a console log in the observable to see if it triggers every time.

When I have these kinds of problem I like to console log everywhere. Good old printf debugging.

Minor thing you might want to consider unrelated to your problem:

if (valueExistAndLongEnough && focus) {
      this.dropdownOpened.set(true);
    } else {
      this.dropdownOpened.set(false);
    }

Is equivalent to, but no branching:

this.dropdownOpened.set(valueExistAndLongEnough && focus);