Google seems to be testing a more defined Material Design Search UI style, according to a user on Reddit who has posted a screenshot of what the new UI looks like. It's not entirely clear when the testing for the design started or how large the testing groups are, but looking at the image, which you can view below, you can see the more defined Search results as easily viewable cards as opposed to just smaller lines that are being used to show the separation of the results.
Since it's unclear how large the testing bed is and how long Google has been testing it, it's also unclear when Google plans to push this out to more users. After some searching using the Google Now search page and in the Google app, the same results as in the image below surface on a Google Pixel, but not all users are seeing the style that's being displayed, so Google is pushing this design for the Search results layout to a number of users. Another difference between the current in-use design for the majority of users and this new one shown below is that the background color of the page matches more closely with the background color that sits behind the Search bar up at the top, and is more of a gray while the cards are white. This is compared to the all white background.
Changing up the design of the layout for Search results may be a minor change but it's one that could be considered an important one, given that it is user-facing and the look of the Search results layout is something that users will have to stare at every single time they open up Search to look stuff up and find the information they want. It isn't the only set of changes though that Google has been working on recently. It was discovered not too long ago that Google was experimenting with a feature called Word Coach which would attempt to help users get a better understanding of words that are similar to each other, though this is also only viewable by a smaller number of users it seems.