Hi, Marian 

Thanks for reaching out to us

Could i take a remote look at your WP-Admin?

It seems that something is blocking the table changes from being saved, once you merge the cells -

but i am not sure yet - don't wish to guess too much, before checking a couple of details.

I will show you a screen recording from my Test site - sending it as attachment;

i am not able to replicate the issue, for us it is saving merging of the cells.

Please provide me a temporary WP-admin (administrator) user for your site where this happens, so we could log in and take a look ‘from the inside’ as that’s the most efficient way to see and resolve the issue. 

We do not interfere with any data or anything else except for the plugin (in case that’s a production version of the site), and of course, we do not provide login data to third parties. 

You can write credentials here just check PRIVATE Reply so nobody can see them except us.

And please point me to the table ID.  Thank you