Thanks for reporting this — it was a genuine rendering bug and has now been fixed.
The footnote marker area had a fixed width that was sufficient for single-digit labels such as [1], but could cause multi-digit labels such as [10] and [11] to wrap under some Linux font metrics. The renderer now sizes the marker column to fit its widest label and prevents the label itself from wrapping, while keeping the footnote text properly aligned and wrapped.
The brackets are valid Markdown and do not need to be removed. I’ve also added a regression test covering multi-digit footnotes so this should not recur.
The fix will be included in the next Objo Studio release: https://feedback.objo.dev/bug/822