In this case there seem to be three bad design decisions: systemd rejects usernames starting with a digit even though they exist in the wild; systemd ignores

In this case there seem to be three bad design decisions: systemd rejects usernames starting with a digit even though they exist in the wild; systemd ignores config lines containing values it considers invalid, rather than failing the whole unit; systemd defaults User to root, rather than making User required. Any of those might be defensible on their own, but the combination is horrid.