You need to use an older syntax because the reporting system is using QtWebKit 2.3 (it's 10 years old).

Source

.foo {
    display: -webkit-box; /* wkhtmltopdf uses this one */
    display: -webkit-flex;
    display: flex;
    -webkit-box-pack: center; /* wkhtmltopdf uses this one */
    -webkit-justify-content: center;
            justify-content: center;
}

There should be a way to use the new syntax on Odoo but I can't help you, if you find a solution let me know.