As Scott said, it’s hard to tell, because the forum ate your whitespace (and we cannot see your EOL characters in a paste).
I did an example: turn on View > Show Symbol > Show all characters to show the different results and make sure spaces, tabs, and EOL were all visible:
Imgur
On the left, I marked \n (LF SPACE SPACE) {top left}, and then replaced it with tab {bot left}. As you can see, it found two LF that were followed by two spaces – one in the WINDOWS EOL (CRLF) section, and one in the LINUX EOL (LF) section. But when it does the replace, it leaves the CR in the WINDOWS EOL section, which Notepad++ properly renders as a newline sequence for line purposes.
On the right, I marked \r\n (CR LF SPACE SPACE) { top right }, and then replaced it with tab {bot right}. As you can see, it only found the one in the WINDOWS EOL section, but properly replaced the full EOL sequence, rather than just the LF portion of the CR LF.
So, if your file has linux line endings, then your regex should have worked for your described data. If you file has windows line endings, then your regex only replaces part of the EOL, and you should use \r\n (CR LF SPACE SPACE) instead.
If you switch to regular expression mode, then the magical \R is enabled: this magical regex sequence matches CR, LF, or CR LF. So search = \R (backslash big-R space space) and replace with \t would work for either style of line ending.