To anyone who ends up here by a search: my problem was that I had previously imported this object using the individual sprites method, and then tried to use the automatic re-import to re-import it using the spritesheet method. This worked for replacing all the objects, but it did not generate the required "associate collision box item with spriter object" events that are required in the construct editor.
I renamed the file, re-imported it as a new object to check exactly what events should be being generated, then fixed my old import to add these needed association events, and now it works great.