Unfortunately, I can’t answer your exact question. This is a known issue #1880 which also affects footnotes. For now, I think most people worked around this issue by either just omitting the reference in the outline or by using an alternative caption (without references) for the outline.
Remove the reference:
#show outline.where(target: figure.where(kind: image)): it => {
show cite: none
it
}
#outline(target: figure.where(kind: image))
or alternative caption for the outline: How to have different text shown in figure caption and in outline? - #3 by flokl