Timeline column header custom date format

The only way I’ve found to customise the date format on the timeline headers is through the #day slot, nothing else seems to work so far.
But when I center the text like so:

        <template #day="data">
            <h2 class="p-2 text-center font-bold">
                {{ formatDate('DDD D MMM', data.date) }}
            </h2>
        </template>

In combination with passing an array of slots:

[{"id":1,"uuid":"0199374e-6003-70ac-bcbd-774ac0d896d4","name":"Morning","active":true}]

The first column header has a bug, rendering the text twice:

Am I going about this the wrong way? Or is this a genuine bug?
The bug isn’t there if I don’t pass any slots