Please make the full backup of your site as well as the Theme settings export and check the issue on any free theme to detect if our theme causes the problem.
I’ve just tried what you told me to do, it doesn’t work on the free theme.
And I set up Woodmart theme on my local computer with LOCAL(https://localwp.com/).
It had the same result.
I found that some post pages of the installed woodmart templates used <!–more–>tags in texts, such as “The big design: Wall likes pictures” of Demo version “Fashion Minimalism”. Those tags didn’t seem to work.
Yes, it means that our theme does not influence this tag working. It seems that this option has been depreciated. I am not sure. Please check the WordPress forum.