Apologies for this very late response. I have been looking into this from time to time again, but still have not found a reliable way to reproduce the issue. So far, it seems like the engine arbitrarily performs a (de)serialization and then the plugin looses certain references.
If you could somehow provide me with exact steps to do in order to get this issue, it might help me find a fix. I would also be interested in your hardware (especially CPU and RAM) to see if the bug is related to limitations in physical resources.
So far, one thing you could try is go to the file addons\assetplacer\placementController\SurfacePlacementController.cs and in line 102 change `editorInterface` to `EditorInterface.Singleton`. If this fixes the issue, please also let me know, so I can include it in a hotfix.
In the meantime, I will keep trying to find a fix, whenever I have the time.