There are a lot of cases where nodes in the material editor will no longer get deleted even if you save the material after deleting the node. As soon as you close the window for the material editor the nodes come back and the thumbnail updates to reflect this.
The repro steps are the most basic example of this however you can run into this issue with many other nodes and it appears that just deleting nodes within the same tree causes issues.
Did NOT occur in 4.15.2 CL 3416026
Found in Binary Preview Build 3___ 4.16 CL 3427662
The scalar node comes back and isn't deleted.
The scalar param node is deleted.
Head over to the existing Questions & Answers thread and let us know what's up.