I was looking for a reference to reply on a thread and found this article: https://narenin.blogspot.com/2011/08/definition-of-pseudo-cursor_09.html
It had exactly what I was looking for and the style seemed familiar....

SELECT @Var += SomeExpresion FROM SomeTable;
is a shortcut to
SELECT @Var = @Var +SomeExpresion FROM SomeTable;

This is a common shortcut in many programming languages. I learned it...

Here's an alternative version because it seems that your numbers are not digit-only.
UPDATE Inventory
SET ItemNum = SUBSTRING(ItemNum, PATINDEX('%[^0]%',ItemNum), 4000)
WHERE ItemNum like '0%'

Instead of using exec (@fullFiles), use: PRINT @fullFiles
Then copy the code from the messages tab and find where you need to fix your quotes. You have some missing quotes...

Reported as spam

I'll be extremely happy the day that JC admits he's wrong in a forum post. Every time he's proven wrong, he just avoids the subject and posts more unrelated nonsense....

I've seen the behavior and it's mostly a display refresh issue. I usually fix it by scrolling the query.
Remember that even if you have a good computer, SSMS is...