You can do this without selecting anything by doing a wildcard replacement. In the Replace dialog, click the More button and check the box for "Use wildcards". Then enter this expression in the Find What box:
(^13[0-9]{2,}) - ([A-Za-z])
and this one in the Replace With box:
\1^t\2
Click the Replace All button, and it's all done.
The Find What expression says to search for a paragraph mark followed by two or more digits, a space, a hyphen, another space, and a letter. The parentheses around the first and last parts make them correspond to the codes \1 and \2 respectively in the Replace With expression. All that changes is the space-hyphen-space replaced with a tab character.
The reason for the problem you had is that internally, Word sees the column of selected text as a series of discontinuous ranges, one in each line. Although there are a few things Word is programmed to do with a discontinuous selection, such as changing font colors, replacements cannot work with it. That's why the selection is expanded to include all of every paragraph that has a partial selection.
If you want to know more about wildcard searches, see https://www.gmayor.com/replace_using_wildcards.htm .