I'm having the exact same issue as some others here and that's the reason I came to this forum...
Backed up iPhone 8 Plus on 13.1 beta (forgot I went to 13.1) and when I connected my 11 Pro Max it said the software was too old. Now I'm currently having to download 13.1 Beta 4 to a half set-up 11 Pro Max so that I can restore it to factory and install my iPhone 8 backup.
Hopefully it all works well and is worth it.
Backed up iPhone 8 Plus on 13.1 beta (forgot I went to 13.1) and when I connected my 11 Pro Max it said the software was too old. Now I'm currently having to download 13.1 Beta 4 to a half set-up 11 Pro Max so that I can restore it to factory and install my iPhone 8 backup.
Hopefully it all works well and is worth it.