What a slack strata manager you have.
Of course it’s a strata concern. The council is unlikely to approve the change without strata approval – so how can it not be a strata concern? Obviously there is going to be increased use of common propery with the maximum number of people allowed in a unit based on the number of bedrooms.
Make your objections known to the council … and start looking for a strata manager who doesn’t object to doing a bit of work for their money.