See https://www.mediawiki.org/w/index.php?title=Parsing%2FReplacing_Tidy&type=revision&diff=2116958&oldid=2116921 .. That effectively mangled the table which I fixed in the subsequent edit using the source editor. The problem seems to be that the serializer didn't know to use HTML tags instead of wikitext tags for that edit.
However, a previous edit seems to have gone fine ... https://www.mediawiki.org/w/index.php?title=Parsing%2FReplacing_Tidy&type=revision&diff=2116921&oldid=2115874 .. so, once selser identified the edited parts and handled it off to the normal serializer, the HTML-table context was missing, it appears.
To be investigated.