There are a couple of things at play here kicking you in the butt.
Firstly, when you have word wrap enabled as you do here, pressing Home (or Cmd+Left on MacOS) you jump to the start of the current "logical" line and not to the start of the line overall. Presumably that's to make it easier for actually editing the content of a wrapped line without potentially jumping farther upward when you meant to go left.
However, while text is selected, you can press just Left or Right with no modifier to jump the cursor to the appropriate end of the selection, which in your case would put the cursor to the left the same as the above sequence, except that since it's using the selection and not the line the cursor ends up where you intended.
If you do that you'll notice that you still end up with a bit of a problem; as outlined in your video there are a couple of blank lines under the text, and doing a Select All selects the line end on line 18 as well as all of the other lines above it because a blank line is still part of the content. When you split the selection into lines that leaves a cursor on line 18, and pressing left moves it backwards one character, putting it at the end of line 17.
That won't happen if the file ends with a single blank line (or no blank lines at all); to get around that you need to manually shorten up the selection to not include the blank line before you split it into lines, or select all, split it into lines, and then do Cmd+Shift+Left Click (Alt+Left Click on Windows/Linux) on the cursor on line 18 to remove it from the selection before you take further actions.