The time taken for app review is very variable. I am not Apple, but from observation, it depends partly on how thorough a review they are giving the app this time and on how busy they are.
In the old days one used to have to allow the best part of a week for the app review of an update to an existing app. Then something in the process must have got streamlined, because one or two days became the norm. But it has never been possible to assume "24 hours".
For new apps the review process is usually longer than for updates. It may also be longer depending on the exact features that the new app uses – for instance, if in-app purchases are involved.
So almost certainly nothing sinister is going on in your case. Wait another week, and if there is still no sign, ask the developer support people to check. Only once in the entire history of the App Store did we have an update "lost in the system", and that was because we had implemented subscriptions for the first time, and there was a policy question that someone needed to sort out, and for one reason another he didn't get on to it until pushed.