e.g. are they staying in the ODS file, and are they then visible in the XLS file when opened in Open Office, and are they visible in Excel if you have it?

Nope! They are only visible in the ods file. Well there are a couple of charts visible, and I thought about this: they are ones that I made fresh, rather than copy-pasting from a different, compiled-data ods file that I had been working off of. Those are the ones that mysteriously vanished. The fresh ones, internally created, persisted.

You know I feel less anxious just talking through this, so thanks. :)