I use Chrome (problem also appear on Mozilla) but it can not be issue just with my browser because when I check site speed on different sites it sometimes show not centered thumbnail (example below from gtmetrix)
https://ibb.co/sJWg973
That also happens sometimes when i check how site look like from different geolocations. That tells me that is not problem with my browser.
And this happens like I say occasionally not every time if you open 10 times different articles this problem will appear to you
I even try with anon surf site and after open few article problem appear (image below)
https://ibb.co/pKstmW4
When I disable W3 Cache plugin the problem is still there. So that is not issue with cache plugin. I use instructions from your site to setup W3 cache https://docs.presscustomizr.com/article/178-how-to-setup-w3-total-cache-plugin-with-the-customizr-theme
Maybe it is some conflict between cropping and centering thumbnail and load on scroll at same time? Just guessing
Now I try to disable load on scroll and that seems to solve problem.
I want to speed up my site so I used this load on scroll. For now I disable load on scroll.