I’m not sure if it would help, but maybe download and reinstall to a more recent installer version.
Perhaps it is also OS-dependent?
I can’t reproduce this on MacOS, Obsidian 1.5.12 (installer 1.5.12) or Obsidian 1.6.1 (installer 1.5.12). Pressing shift-down selects only the first line and no portion of the second line. Deleting deletes only the first line.
(I’ll just sneak in one quick workaround too. Delete Paragraph command would also delete the line without needing to Home-Home-Shift-Down-Delete. Can also be mapped to a hotkey.)