There must be something else deleting those nodes from Firebase. Check to see that you don’t have any stray blocks assigning values to Firebase paths. Is anyone else you know using the project or a copy of the project that can access Firebase?
If you can’t figure out the cause, start to slow things down. Add wait blocks so that you can watch the Firebase values to see when they change. It helps to do something visual on the screen like change a label’s value right before the wait block so you connect that to what is happening in Firebase. This is the method I use for debugging blocks: Debugging in Thunkable X (Video).