Ok so I tested this again on this site:
http://www.affordablehomesa.co.za/
Was thinking last night, perhaps I did not clear cache properly after testing it on the solar site.
First I disabled Joomla Cache.
The result was, form returns back to itself with no warnings. If I refresh the page, then the warnings come up, even comes up on other pages throughout the site even though I did not submit the form on those pages.
2nd, disabled minify css and js.
Result, the same.
Then I disabled Fabrik cache globally, result the same.
Then I physically disabled the System Cache Plugin.
The result was the same, with the difference, even if I keep refreshing, the warnings still don't come up.
I think it's possibly safe to say, this is not a cache issue. After every adjustment, I also cleared the Joomla cache. PageSpeed is also not enabled on that site.
*edit, just out of interest, long ago already i gave up on having the Ajax form validation switched on on any forms as I just could never get it to work.
Another point I'd like to raise is, on this Affordable home site, it's all Helix3 template, I did not use PageBuilder for any of the pages. And it's all Fabrik module, no Modules Anywhere.