This is indeed a weird one! I was able to get it in my starting list only twice (out of many trys), even though it was marked as "Complete" in my Tech Library. We have a save of it happening though, so hopefully the devs will narrow it down.
My custom civ has Navigators as a perk, so this has happened to me.
I never reported because it did not matter as one of the other if not all of the techs available were ones I needed to research, and it was never there after the first tech was researched.
I believe I remember exiting Research doing other things and coming back before end turn and that tech was not there.
Usually, I forget to assign a leader to Tech so have to come back when I have, and this tech disappeared.
A worse problem is reports of having researched the Tech and it not triggering although some tech/other "improvements do not trigger on the turn finished but the next turn.
This thread. https://forums.galciv4.com/516029/bugs