I've talked this over with the team today and we're going to tackle this.
What needs to be done is that the Flarum Subscriptions extension needs to also listen to the PostApproved event so that it dispatches the notification to users. It is a straightforward fix, but the foundation will compensate the respective core developer for the time regardless. Issues like these are annoying and reporting them helps us understand their priority.
The ability for the foundation to invest in these matters highly depends on the amount of donations we get. Pledging (which I enabled on this issue now) and becoming a supporter/sponsor are fundamental sources for this.