There is one more occurence of CSS I forgot to mention. To prevent the said tag from showing in the affected discussions above the title, I used this CSS rule:
.TagsLabel .TagLabel:not(.colored) {
display: none;
}
But be aware that this hides all uncolored tags – i.e. all tags without an assigned color code.
So, you will end up with:
.Dropdown-menu > li.item-tag99,
.TagsLabel .TagLabel:not(.colored) {
display: none;
}