At least one WebCompat issue has been found related to this, as seen at https://webcompat.com/issues/110041 I believe this is covered by the web platform test css/css-break/widows-orphans-017.html Chrome and Firefox now pass the test, as of https://bugs.chromium.org/p/chromium/issues/detail?id=1156312 landing.
Hello Thomas 👋 Thanks for the report. https://wpt.fyi/results/css/css-break/widows-orphans-017.html Probably something to dig around https://searchfox.org/wubkat/source/Source/WebCore/rendering/RenderMultiColumnSet.cpp
<rdar://problem/107689079>