strange behaviour in LibreOffice Calc save to CSV

[Gary Dale]

If I open a .csv file containing e-mail addresses then save it back as a 
.csv, underscores and "@" get translated into a character code. It does 
this whether I use save or save as.

However, if I check the box in the save as dialog to edit the filter 
settings, it saves properly, even if I don't change any filter settings.

It's only recently (for the last month) started doing this. Before it 
saved the characters as themselves rather than codes.