@smnokars While Notepad++'s behavior has changed you can get exactly what you desire with a small change on your part.
Step one remains the same. Use “Alt + Mouse” and “Ctrl + C” to create the selection and to load it into your copy/paste buffer.
For step two though use Alt+left or regular mouse click at the spot where you want to paste the selection. This will move the text cursor or caret to the indicated spot. Now you can do Ctrl+V to paste and you will get
123456789_1 123456789_1
12345678 12345678
123456789_12345 123456789_12345
123456789_123456789 123456789_123456789
You don’t need to create the skinny multi-row selection that matches the number of lines in the paste buffer that you were doing in step 2 before.
Older versions of Notepad++ also worked this way with using Alt+left mouse click to position the text cursor or caret.
The behavior change with multi-row selections has to do with what the developer calls multi-caret mode that was introduced in v8.6.
If you have been using column selections then may want to go to Settings / Preferences / Editing 2 / and to turn off Enable Column Selection to Multi-Editing. That’s one of the new v8.6 features that v8.6 enabled by default. Turning it off restores some of the older behavior though apparently does not fix the issue you brought up here.