Steps to Reproduce:

1. Create a world in creative mode
2. Give yourself a structure block
3. Save a structure of a tree by placing a sapling and use bone meal to grow it and typing in tree as the structure name
4. Load the structure
5. Type in /structure delete tree
6. Remove the structure block and place another one, and then go into load mode and type tree.

Observed Results:
The structure "tree" is not removed, as it is still saved and it continues to be that existing structure.

Expected Results:
The structure "tree" should be removed and the structure block should be unfamiliar with the name of the structure if it is in load mode.