Index Card Text Changed/Corrupted

I have not seen this behaviour before.

I opened an older file and updated the file to reflect the newest version of Scrivener. I began working in the outline, with some minor edits and then discovered that text in a few of the index cards had turned into gobbledy-gook, with seemingly no way for me to undo that.

Can anyone offer me some advice on what to do about this, please?

Thanks