Looks like Google is testing having those sub-navigation links in search, in the bubble format, stick to the top of the search results as you scroll down. We kind of saw this without the bubble header format in December but here it is with the bubbles buttons.
This was spotted by Mordy Obserstein who posted a video of this on Twitter. Here is a screen shot:
Here is the video:
— Mordy Oberstein (@MordyOberstein) February 14, 2021
Brian Freiesleben spotted this a few months ago as well:
Looks like Google is possibly testing some minor UI changes for entities/knowledge panels in the mobile SERP. The refinement bubbles are sticky as well (last screenshot). pic.twitter.com/r4uo3GT5ea
— Brian Freiesleben (@type_SEO) November 1, 2020
Forum discussion at Twitter.