I'm stabbing in the dark trying to remember this, but could this be something to do with the Preference 'Store Presets With Catalog" in the Preferences? I vaguely recall someone losing presets when they exported a catalog, but like I said, I'm a bit hazy on this one.
Originally if there was a malformed Preset, Lightroom would give an error about switching modules. This was related to presets that had been hand edited with typing errors. I think the team did change the handling on this, but I've not tested with by making a deliberate error (but I may just do this anyway!)