Please note that LENGTH is a text field that has very little to do with the actual length of track. You can write more or less any value into that field without any influence on the length - even a text like "this is the length".
A output can only be generated if there is any contents for a field. A field whithout contents does not exist.
If you refer to the file property length as written by the encoder, then use always _LENGTH.
I think that this should also be _TAG.
Check where you see
If that is the column in the file list, check the column definition which may have the label "Tag" but probably has set as
Value: %_tag_read%[ (%_tag%)]
Which shows you which tag is read and which tag versions are present in a file. _TAG then shows you all tag versions, e.g. ID3V2.3, ID3V1, APE.