Website is deranking on SERP, is it Webflow fault?

The project has been active and on CMS hosting for some time now, and over time we have had good positioning on SERP in general, but, every several days (5-7) (for the last 4 weeks, today again) we have a huge drop in SERP positioning for all keywords where we have dropped from top8 to bellow100, and then after few days it’s back in top8.

From how the page is set on Webflow, I can not see that the Webflow is causing that SERP drop, but the client SEO agency is determined to blame Webflow for it.
Their reasoning is that due Webflows CDN delivery of .css and .js
causes Google to reindex all the time, as well as the size of the webflow .css file also as reason Google is slow to render site in GSC.

I asked Webflow support for help also, but so far no response.

here’s the read-only link to a project:

can anyone help?