Apparently it is meant as an easter egg/announcement of the new stacked pull requests feature (https://docs.github.com/en/pull-requests/how-tos/stacked-pull-req

Apparently it is meant as an easter egg/announcement of the new stacked pull requests feature (https://docs.github.com/en/pull-requests/how-tos/stacked-pull-requests https://docs.github.com/en/pull-requests/how-tos/stacked-pul...), but with GitHub's gradual quality degradation and nothing denoting that in the tooltip it just looks like their icon font is malfunctioning.