My test blog is running WP 5.0-beta3-43867 with Gutenberg 4.3.0-alpha-0841246 (from Daniel Bachhuber’s nightly), and the theme I’m using is Cover2 (Github here), which also appears to exhibit this in single post view.

I’ve narrowed it down to this function (called from header.php), where the gutenberg_parse_blocks() call triggers the issue.

I’m not saying I’m 100 % sure it’s Gutenberg and not the theme(s) at fault here, but I just thought I’d post this just in case it helps to find out either way.