1 - Action to copy the feat artist from artist to title:
Format tag field
Field: title
String: %title% $mid(%artist%,$add($strstr($lower(%artist%),'Ft.'),6),$len(%artist%))
2- Action to cut the feat artist from artist
Format tag field
Field: artist
String: $left(%artist%,$strstr($lower(%artist%), Ft.))
3- a little hygiene
Format tag field
Field: artist
String=$trim(%artist%)
If am pretty sure that
%title% $mid(%artist%,$add($strstr($lower(%artist%),'Ft.'),6),$len(%artist%))
will not work as you have a Ft with a capital to compare with but the ARTIST is always in lower case (see "$lower"). So the result will always be 0 - which means the whole string.
Also, the "6" in that expressions is the count of the letter f-e-a-t-.-blank as the featured artist will start behind this string. If you reduce the "feat. " to "ft." you would have to set a 4 instead of a 6.
I tried your changes and now it works on the tracks with ft. in it but the tracks with out it, it removes the ARTIST.
This is what I am using
1 - Action to copy the feat artist from artist to title:
Format tag field
Field: title
String: %title% $mid(%artist%,$add($strstr($lower(%artist%),'ft.'),0),$len(%artist%))
2- Action to cut the feat artist from artist
Format tag field
Field: artist
String: $left(%artist%,$strstr($lower(%artist%), ft.))
3- a little hygiene
Format tag field
Field: artist
String=$trim(%artist%)