I don’t know where to report this error.
That’s caused by Previous search results are not cleared when searching (#230) · Issues · GNOME / gi-docgen · GitLab. Not sure how when that fixed version will be used for the docs hosted on docs.gtk.org though.
@ebassi @matthiasc do we need a new release of gi-docgen to get this fixes into builds, because they use a stable version from Fedora?
Possibly; I am going to do a release today.