I think a third list is needed so you can compare if items got deleted from one list. It serves as current state to which you compare the changes of either list, so you know which entry is either new or deleted.
@Vergil365 @jmaguire I gave up on the blueprint and created my own automation to sync my lists. Always had items that didn’t get synced or showed up multiple times. For my bring and Google keep sync it works since a month now without any issues, I can share it if you want.
Edit: For those interested, here is my todo list sync blueprint.
I do capitalize as well, but after the capitalized synced entry gets added, it doesn’t sync it back to the originial list, where it isn’t capitalized. If you need the entries to stay lower case, I could change it for you, so they stay lower case.