If you enable an experiment feature such as Spectator mode or game test and save a structure after you save and quit the structure gets removed.

Reproducing the bug.

1. Open a Minecraft world with any experimental features enabled.
2.  Save a structure using the /structure command.
3. Save and quit
4. Re-open the world
5. Try to use /structure load.
6. You will receive the error message saying no structure by that name exists and the structure will not load.