On April 6th, Google said it made an update to “the algorithm for counting image impressions in Search results.” This might lead to a “Small drop” in the image impressions you see reported in the Google Search Console Performance Report.
Google posted this information on the data anomalies page where Google wrote, “Search Console improved the algorithm for counting image impressions in Search results. As a result, you may see a small drop in your image impressions in Search. This is not a drop in actual impressions, just a more accurate calculation of image impressions when type=image is specified in the Search Performance report.”
So you will only potentially see this start on April 6, 2021, specific to when you filter your search traffic by image search.
Forum discussion at Twitter.