In our PSI data we are seeing some pages with outrageous CLS numbers which we cannot reproduce or make sense of. In fact, here is an example of one page that has 2.52, but I did not even think it was possible to get over a score of 1.0, which would be a complete shift of everything on the screen, am I right? And is there some problem with the data/chrome, because this is not an isolated incident...our site pages suddenly started suffering terrible CLS data about a month ago and we are bewildered, in Core Web Vitals area of GSC.
Look in the Field Data at the CLS...2.52, but the lab data is .044. PSI Link
CLS in the lab tests (synthetic tests) is purely for initial page load and above the fold.
CLS in the field data (real world) is measured from the second the first (well technically second) paint event right until page unload.
So if there are layout shifts happen as someone scrolls the page those keep adding to your CLS.
Imagine you scroll the page and the scroll bar suddenly appears, that would shift the whole page. Now CLS is based on the percentage of the page that moves. So if the whole page shifted to the left by 10px you would get a Layout Shift of almost 1 (think of 1 as 100% of the visible page, 0.5 would be 50% of the visible page moved etc.).
Let's assume that as you scroll the page further the scroll bar suddenly disappears, the whole page now shifts to the right by 10px. This would result in an additional Layout Shift of almost 1.
Now you have had two Layout Shifts of almost 1 - your Cumulative Layout Shift would be almost 2.
I have simplified how layout shift is calculated but I think the principle is easier to understand with the above example.
As for CLS data suddenly changing, I would recommend using something like the web vitals library to pipe the data to either a custom backend or to your analytics so you can see if this is a specific device, screen size etc. causing it.
To see layout shift regions, go to Developer Tools - > Rendering -> Check "Layout Shift Regions" and then load the page a few times, resize it etc.
The only thing I could see is that your mobile menu has some very strange layout shift regions that are particularly bad at large screen sizes. Other than that there is a massive shift when the page loads but that shouldn't take it over 1.
I know the problem is on desktop but I can't remember if they put tablet data in the desktop or the mobile field data...if it is desktop then you may have your answer!