Wikimedia Bugzilla is closed!

The rule that ensures vertical centering:

.mw-hiero-table td {
    vertical-align: middle;
}

...is overridden when hieros are embedded in another table, such as infobox, which have their own vertical-align set. This is because rules set in Common.css override those in extension CSS (by being loaded later. A simple fix is to add specificallity, thus the rule should be changed to:

table.mw-hiero-table td {
    vertical-align: middle;
}

This ensures that blanket CSS rules (classes not attached to elements) do not override the extensions' CSS.