OK, I can reproduce it, but exclusively when using the link modal, right? (Just to confirm the scope of the issue)
If I paste it directly: https://en.wikipedia.org/wiki/Horncastle_boar’s_head ; or paste the URL on top of a selection, it works as expected.
EDIT: Or… almost. The direct URL gets decoded: https://en.wikipedia.org/wiki/Horncastle_boar’s_head, which is also unexpected and avoids inline oneboxing.
Thank you for the report!