Switching off Unicode use in MP3Tag using the appropriate option would prevent this, but (AFAIK) would lose the use of Unicode in all of the other fields as well, which would be a Bad Thing.
Although Microsoft seem to be clearly wrong in doing this, it is questionable whether there's any advantage in having a Unicode TLEN, since this field can only ever contain the characters "0" to "9" (any comment, Florian?).
Files that have had their TLEN field written as Unicode by MP3Tag (and are therefore unplayable by WMP) also have (very minor) display issues in Windows Explorer.
In a quick test, if the columns "Audio Sample Rate" and "Channels" are selected for display (right click the column headers to do this), they are blank for affected files. Other fields may also be affected - I didn't populate every possible field to test them. The fields that are affected may also depend on the order in which the frames are stored within the ID3 - it's possible that Windows Explorer just gives up processing when it encounters the invalid field.
This isn't a major inconvenience, but does demonstrate that the effects of Microsoft's "tightening up" on non-compliant ID3s extend beyond WMP.
In another sense, it might be useful, by providing a way of detecting which file are affected, without having to open each one.