Post shows wrongly formatted – in preview? No

I’m getting rather desperate because our latest blog update is long overdue. The update was ready to post on Monday, our readers are used to at least one blog update a week, now its Friday!
My support request https://www.ads-software.com/support/topic/urgent-post-formatting-messed-up-with-version-3-2-4/#post-8237067 still hasn’t been answered. In the meantime I have had a lot of stress and have wasted more hours on (not) solving this than it would have taken me to write the next blog update (so I’m behind with my work too!).

The issues:
in preview the post shows

1. without it’s <h1> header
2. the font size is .single .format-status .entry 24px, it should be 18px

If I risk posting this without correction the post will go straight away (most likely in the wrong format) via rss feed to several sites who link to us. Also Google will index it, possibly without any <h1> header (post title) – which will be bad for SEO. (60+% of our traffic is from search engines.)

You see: I really need this fixed!

In the meantime I also updated to Hueman Version: 3.2.8 – that didn’t fix my problem! I deleted the theme completely, re-installed it afresh – that hasn’t fixed it either! I deactivated my sharing plugin (which might have been one cause) – that didn’t fix it either! There is no single.php in my child theme (which could be another reason) – so that’s not it! I’m truly at a loss…

What I still don’t understand: why is this only affecting my new post and NOT my older posts?