It’s not the strata management company thst charges interest, it’s the owners corporation. The Strata manager just collects it on behalf of the owners.
The decision not to charge interest needs to be resolved at a general meeting. The rescinding of thst motion would also require resolution at a general meeting.
Provided the original decision to not charge interest was passed at a general meeting and it was minuted, then it remains in play.
Go back to the minutes and find the decision, otherwise the strata manager is complying with the act.