Made a dictionary of wave information resources in the level.
I exported a dictionary of custom WaveInfo resources in my level scene, and went to add a new resource to the dictionary. I typed in the key, added a new WaveInfo, and clicked Ctrl+s.

My new key-value pair promptly vanished without a trace

And so began the hour of removing variables, re-adding variables, trawling through the Godot forums and generally wanting to set fire to my computer to try to find the problem. The urge that was not improved by the discovery that I’d forgotten to press the button to actually add the key-value pair, and the error firmly existed between chair and keyboard.