This is not a bug. Some field is actually truncated every time this warning occurs (not necessarily the field you're actively setting though).

The reason that you can't see the truncated ID3v1 fields in the browse tool is the "Intelligent tag-sync checking mode" setting (in the "tagging core" settings). Please read more about it in the help, and try to turn it off and you'll see what I mean.

If you don't want these warnings at all, you can turn them off by disabling the "Warn when ID3v1 tag fields are truncated when set" setting (in the "General tagging and renaming" settings).