Convert filename puts new tag in Track Number column not Title

Not sure why this happens?

Also not sure.
You would have to show us the format string and the real life filename.

Here are screen shots

A filename like
01 This Wheel's on Fire.mp3
has the track number at the front but then follows text.
So it is at least 2 parts. So you also have to have 2 parts in the format string.

If you really only want the number, then use as
Format string: %track% %dummy%

But as I see that the TITLE field is also empty, you could fill that and TRACK in one go:
Format string: %track% %title%

Thank you - makes perfect sense!