Unwanted modification of metadata

Hi,

Long time RB user, I'm converted my CD collection in oggs, which I manually tag using Quodlibet.
I've been updating the tags of many old files, with RB closed.

I've recently noticed that RB is trying to change the metadata of the edited tracks behind my back at startup... 
It produces "Error while saving song information Could not open resource for writing." messages if my directories are read-only, and otherwise my tags are rewritten with the tag names in uppercase, the date in "YYYY-01-01" format, and the tracks are seen as corrupted by ogginfo.

Running in debug mode, I see (summarized):
[rhythmdb_process_one_event] rhythmdb.c:2668: processing RHYTHMDB_EVENT_FILE_CREATED_OR_MODIFIED
[action_thread_main] rhythmdb.c:3019: executing RHYTHMDB_ACTION_LOAD for "file:///...file with modified tags"
.....
[rb_metadata_gst_load_tag] rb-metadata-gst.c:444: processed string tag "..."
.....
[rb_metadata_dbus_load] rb-metadata-dbus-service.c:128: metadata load finished (type application/ogg)
.....
[_handle_message] rb-metadata-dbus-service.c:353: handling metadata service message: save
[rb_metadata_save] rb-metadata-gst.c:1014: saving metadata for uri: file:///...

So my question: why would RB try to rewrite the tags of a file which has changed?

Thanks for any clue,
Omen.