The closest explanation that I can see is Why is a `rowspan` cell with `colspan` equal to number of columns seemingly only spanning one row? - #4 by PgBiel. Perhaps @PgBiel can describe more in depth or correct me.
I seem to recall a forum post or GitHub issue about a similar issue (perhaps about heading or list/enum numbering?) but can’t seem to find it.
It seems clear to me that numbering is applied from the first non-row-spanned cell and then carries on.