www.wysiwygwebbuilder.com

Sometimes, after publishing I find the content in (random) dozens of my published pages have become 'centred'.
The content as seen in WYSIWYG Web Builder is correctly left justified (apart from a few items like titles).

The only way I can fix the online content is to alter the page is some very small way, and then re-publish.
The pages are not fixed by publishing 'Entier web sit' and 'all pages'.

This is becoming a big issue as I have dozens of pages in the website and the only way I can discover errors after publishing is to visit every page to see it the formatting is correct (which is a right PITA!).

These are some examples (at the time of writing)

Correct formatting
https://www.etlcpp.com/function_tutorial.html

Incorrect formatting
https://www.etlcpp.com/container_tutorial.html

This issue is giving my website a very 'unprofessional' impression to visitors.

Your page has duplicated ID's so that is why the formatting is 'random'.

Related FAQ:
viewtopic.php?f=10&t=48540

I seem to remember that WYSIWYG Web Builder used to warn me when that was the case.
I'm not seeing it any more.

Maybe you have disabled the warning?
Tools -> Options -> General -> Automatically check for duplicated ID's

Or else
Page -> Error Reports

Tools -> Options -> General -> Automatically check for duplicated ID's
That box was already ticked.

Did you try Error Reports?