Fascinating bug; I can confirm its existence, and I want to note that the issue indeed does not appear if and when logged in (as an admin), but it does appear as an anon.

Running the page's HTML through W3C HTML Validator reveals that there are incorrectly nested tags in the Flow board element(s). I'm not sure how Flow initialization works/is supposed to work in ideal conditions, but it seems that some Flow-related HTML is injected before the actual page content container element (div.mw-content-ltr) and this somehow results in the HTML nesting issues, which in turn result into this bug.

Either way, this sounds more like a Flow issue rather than a Timeless issue, given that the skin obviously by default does not have any incorrectly nested HTML. (This can be verified by running https://www.mediawiki.org/wiki/Special:BlankPage?useskin=timeless through the Validator and you'll find that the only issues it complains are missing alt attributes on the logo elements.)