Generating WEBP when previewing in Chrome

So this is weird. I made a background graphic and exporting it as PNG 24, because it’s all vector shapes.

note: I have the “generate WEBP” setting in the project settings turned OFF.

When I preview the page directly in Chrome, the background graphic is loaded as a WEBP which Blocs generated:

So here’s where things getting really weird: If I first use Blocs built in preview, and then preview in Chrome while still in the Blocs preview screen, the PNG is used and no WEBP files are generated:

Please note that when I publish or export, everything is fine and there are no PNGs replaced with WEBP. So it’s only a “preview thing” …
???