I don't know if the following has been mentioned yet, so forgive any duplication.

Recently I was playing a game on a Large map with hard difficulty and epic pacing and I ran into this issue. I was researching Earthly Guardian and with about 7 turns left to go, I received the tech which gave more spell books. I went into the screen to change my research queue, but when I did, my research for Earthly Guardian went back to 30 turns to go. I tried it several times with the same effect. While this issue may be barely noticeable on normal or fast pacing, on epic it was terrible.