I think I solve the issue, but I’m not sure what caused the problem.
In my post, when I created a column and put heading inside the column – it didn’t work.
It started to work, when I created another heading outside the column. – After that both headins worked – the one inside the column and the one outside.
If you want to check if that’s a problem with a plugin, you could make a post and create it like this:
GROUP
>COLUMN
>>HEADING
On my website this set up makes the heading not working.
When I put another heading outside the group and the column, it works fine.