The "15" is scratched out but, the 34 is not displayed. Look at the corresponding wikitext diff which shows that 15 has been replaced by 34.
|mediawiki/extensions/VisualEditor||master||+0 -169||Revert "For empty / whitespace-only headings, output <p> instead of <h#>"|
This looks like a very complex issue related to the fact that MWHeadingNode has the handlesOwnChildren property set (because of a hack that converts empty headings to paragraphs - which should possibly be moved to Parsoid...), and some weird behaviour that triggers it the deep re-application of html attributes in ve.dm.Converter.static.renderHtmlAttributeList. Basically it's all @Catrope's fault :)
So turns out that in scrub_wikitext mode, Parsoid does now remove empty headings, so we should be able to kill that hack. That will fix this specific issue, but I still suspect there is a problem with renderHtmlAttributeList.
The patch was merged some days ago (Gerritbot really doesn't like this task…). The related task mentioned above is also fixed.
All of this code is actually already in production and works correctly: