there is black bars at the sides
you can see it on https://routers.world/
This doesn’t appear to be specific to RTL mode, as it’s not there when in safe mode — it appears to be related to a customization on that site specifically (they’ve applied a custom background color to the sidebar).
They can fix it in their theme, by adding a new style like this:
.sidebar-footer-wrapper {
background: var(--sidebar-color);
}