Feeds are cleared/dumped roughly every 30 days
All feed content data seems lost after about a month, recurring consistently.
That is, the state of feeds (favicons, downloaded articles) is lost, though the content already downloaded (every article evolution contains at that point) remains/is not deleted. However, duplicates are created for those articles already downloaded.
Evolution functions normally until, after about 30 days since the last incident, evolution loads with every feed's favicon missing, replaced with a generic RSS icon placeholder. On clicking "Send/Receive" to fetch the feeds, the feeds are refreshed, and every article is duplicated. Favicons are also updated properly.
If a subfolder exists containing feeds, these feeds are duplicated as separate folders in the main "News and Blogs" folder, whereas feeds that are already located in the main "News and Blogs" folder have their articles duplicated within each feed's folder. This behavior is observable in the attached screenshot: Notice that the feeds listed under the "YouTube" folder at the bottom, have now appeared as new directories under the main "News and Blogs" folder, and that the old entries have not had their contents or favicons updated upon clicking Send/Receive. However, notice that other items directly within "News and Blogs" (such as the highlighted "Blog & Mablog") simply have their articles duplicated alongside the older copies of articles.
The recent list of dates on which this bug has occurred, to demonstrate the consistent timing of this "reset" of evolution-rss (month/day/year):
This has been occurring for some years, so it is not a recent bug with any of the GNOME stack.