Like Instagram have this favicon, and the result in google in good

Yeah i haven't understood it

It half feels like it randomly changes for me

What file extension do you use for favicons?

Google doesn’t use your site favicon as-is.
It crops it to a circle, applies its own background, and caches it. Make sure your favicon is square (48×48 or larger), has transparent background, and the icon itself fills most of the canvas. Changes in Google Search can take days/weeks to update.

https://developers.google.com/search/docs/appearance/favicon-in-search

Im using .ico extension and there is my favicon :

Oh mb there is no ico preview here

anyway, this is a rounded black rectangle with my icon in center. So I have transaprency in corner for rounded corner

i wonder if the act of some transparency makes google think it yeah

But Instagram for example use the same favicon format

Like there is transparency here no ?

oh yeah

though i wouldnt be suprised of google has manually hand picked ones