Google has documented a bug with Google Search Console reporting, specifically the performance report when filtered to Discover data. Google said you may see a dip in your Google Discover reported traffic because a reporting bug, but in reality, there was no real dip in that traffic.
The issue was between December 8th and 13th. Google wrote "Because of an internal issue, you will see a data drop in the Performance report for Discover during this period. This issue has been fixed. Please note that this does not reflect any user-facing changes for your site, only the data reporting in Search Console."
December 8-13:
So if you see a drop in that chart, it might have not been legit. Of course, there is no way to know if you saw a dip there because you cannot track Google Discover traffic in Google Analytics.
Forum discussion at Twitter.
