I think aligning the separator line to the side by default is appropriate in Twenty Nineteen because headings have a similar line on the side.
However, if you purposely set it to align center, that does not work at 768 pixels and wider:
@media only screen and (min-width: 768px) {
.entry .entry-content > *.aligncenter,
.entry .entry-summary > *.aligncenter,
.entry .entry-summary > .wp-block-group > .wp-block-group__inner-container > *.aligncenter,
.entry .entry-content > .wp-block-group > .wp-block-group__inner-container > *.aligncenter {
margin-left: 0;
margin-right: 0;
}
}
The side margins were set to zero before the theme was widely available.