Hi !
There is two problems about this, the first problem is that R3E does not give the coordinates data and so the plugin which computes live delta and such features is not fed with the required data.
The data displayed in the default nextion templates are the one from this plugin. It was not really a concern until recently where more and more people are using nextion, which one is harder to change displayed data compered to 7 segments displays.
To solve your problem you have to modify the "mapping file" in "NextionTemplate" folder to change the PersistantTrackerPlugin by their in game replacement (ie PersistantTrackerPlugin.AllTimeBest should be replaced by DataCore.GameData.NewData.AllTimeBest),it will shortcut the plugin and feed the screen directly with the game data, you will have a partial set of data (Lap history is not available per example)
The task may look a bit hard but if you wait a couple of weeks I will try to address this kind of issues. Like you may have seen I'm working a new plugin to make wysywig dash and it uses the same engine than nextion and so have the same problems. I've opened an issue in the bug tracker so you can follow the evolutions about this :
https://github.com/zegreatclan/AssettoCorsaTools/issues/94