In our case, the problem turned out to be adding an AVAssetWriterInput to an AVAssetWriter with an ARSession already running (which also lost tracking at that point; this is what gave us the clue). The AVPointCloudData asserts went away when we set up the AVAssetWriter first and then ran the ARSession.
So it had nothing to do with the data model, we're not using that.
MarnixArnold
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This is certainly taking its sweet time. On our Feedback Assistant ticket it now says "Recent Similar Reports:Less than 10, Resolution:Potential fix identified - For a future OS update" and that's it.
I would urge anyone experiencing this issue to file a Feedback Assistant ticket of your own. The more people make themselves heard the more likely it is that Apple will treat this with a bit more urgency (hopefully) or at least give us an estimated date or iOS version for a fix.
I would like to add my voice to Chris Heinrich's: this breaks our app as well, to the point that we are advising our users against updating to iOS 15 if they can avoid it, since we cannot use relocalization anymore. We depend quite heavily on relocalization working properly (as it did prior to iOS 15) and we haven't been able to find a reliable workaround. A fix would be very welcome indeed!