Description of the Issue

If one uses the sorting feature with a column-block defining the sort key, and this column-block occurs after a tab character on the lines, sorting doesn't work correctly.
For some additional info, see https://community.notepad-plus-plus.org/post/63877

Steps to Reproduce the Issue

1. Create a new tab with the following data:

	b
	a
	f
	c
	e
	d

Same data shown with View > Show Symbol > Show White Space and TAB enabled:

2. Define a column-block, as shown here:

3. Execute Edit > Line Operations > Sort Lines Lexicographically Ascending

Expected Behavior

Actual Behavior

No change; data is still in original order as shown in step 1 above.

Debug Information

Notepad++ v7.9.4 (64-bit)
Build time : Mar 15 2021 - 01:03:10
Path : C:\.......\npp.7.9.4.portable.x64\notepad++.exe
Admin mode : OFF
Local Conf mode : ON
OS Name : Windows 10 Enterprise (64-bit)
OS Version : 1909
OS Build : 18363.1379
Current ANSI codepage : 1252
Plugins : mimeTools.dll NppConverter.dll NppExport.dll