Normally, all the buttons go to the right, this is why the caption is aligned left.

You can avoid large empty spaces in the caption by using Settings / Tiger / Lightbox / Caption / Adapt to content width option.

If you prefer the fixed width caption area and have the comment still centered, use this CSS "hack" (Settings / Tiger / Advanced / Custom code / CSS box):

.lightbox .lightbox-caption .caption { flex: 1; }
@media screen and (min-width: 40em) {
    .lightbox .lightbox-caption.hasbuttons .caption > * { 
        text-align: center;
        width: auto;
    }
}

Note, this will center the text in the area left to the buttons, not the whole page.