The way Android notifications work is --
- an app "posts" a "notification object" which describes everything about the notification (the LED, text, summary, sound)
- the rest is done by Android
- it isn't necessary to "remind" Android about an already posted notification, in fact the app's OS process may go away
An so, sorry, I don't quite know what might be wrong...
On the other hand yes maybe NextPlus does post repeat notifications - or maybe it's not marked as "designed for Android 6" (internally), and that changes how Android treats its notification (e.g. do its notification icons have color?)