This has now happened for me on 2 documents in a row.

I had a document that had H1 titles throughout the document. I changed them all to H2 tags, and added an H1 one at the top with the title of the document. Below the top H1 title, I added a table of contents starting at the 2nd level. Now all H1 tags are showing up the same at H2 tags (same horizontal line below and same font size). I checked the settings in wiki mark-up and everything looks proper there. I tried creating another H1 tag in another part of the article, but it looks like all the other H2 ones as well. I tried recreating the document by copying and pasting everything from the wiki mark-up, and the same thing happens.

I look forward to reading back on this issue.