‘0’ showing as blank

I have a field which is a text-line field and holds numbers, which could also be fractions. The values could be for example, ‘-1’, ‘0’, ‘?’, ‘1’, ‘1?’, etc. Since a recent upgrade any value which is ‘0’ is just being displayed as ” when I edit the record. However the value is still ‘0’ when I look at the database with phpMyAdmin and it’s also OK in the CSV download.