After upgrading one of my PC's with SSD from 18.04 (kernel 4.15.0-xy) to 18.04.3 (kernel 5.0.0-xy) I had the similar shutdown problem with 18.04.3. Since this problem is not happening in my other PC's with the ''original'' Ubuntu 18.04. (now kernel 4.15.0-72), I made fresh installation of the ''original'' 18.04.0 (instead of 18.04.3) and as a result there is no more shutdown problem in this PC.
Source for Ubuntu 18.04.0 (''original'') is http://old-releases.ubuntu.com/releases/18.04.0/