Hi!
Let's assume that you have tktreectrl widget (treeW) created and you call following code:
set stringToDisplay "foo\tbar"
$treeW item text 5 col1 $stringToDisplay
It can be seen, that indeed item "5" has updated text in "col1", but displayed text is "foobar" instead of "foo bar".
This issue is visible on 2.4.1, but most likely it is also visible on earlier versions.