I am not sure if this is new, it may be from the late 2018 Google change but Google can now let you see all the products within a collection when Google shows a products carousel in the knowledge panel. This also may be more prominent now with the product enhancements in search from a few weeks ago.
Clarie Carlile posted on Twitter that you can now see below the product carousel within a knowledge panel a link to “explore collections” for a specific e-commerce category. Here is a screen shot that I can replicate on Safari on a Mac but not Chrome on a Mac:

When you click on the explore link, it opens up all the products in that category from that site:

I just got a product collection showing on desktop in the business knowledge panel, with the option to click through and see all of the items in that collection. Anyone else seen this? #localSEO @TheSocialDude @JoyanneHawkins @rustybrick pic.twitter.com/GQARbIfccE
— Claire Carlile 🕊️ 🌊 🥑 (@clairecarlile) March 11, 2019

In this case, the products showing in the SERP example were all manually added in GMB in Products (Beta)
— Claire Carlile 🕊️ 🌊 🥑 (@clairecarlile) March 11, 2019
This might be a limited test because not everyone can replicate it all the time.
Forum discussion at Twitter.

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