You most probably have some other mod which is changing hud.gui.

Last mod placed in list of mods which changed hud.gui will be used. So that last mod which change the same file is probably overriding this icon (and returning things to previous state).

But since game version 1.5, moders can add icons (UI elements) without changing existing game gui files. This mod is doing that as well. Just left from that section where you have found encyclopedia icon (left from 3 dots) you can find newly added icon for opening this mod. This icon should be visible regardless if some other mod have changed hud.gui or not.

I did not remove old way (icon you are missing) of accessing the mod. I think it is better placed and more convenient. Also people have been used to that icon. But that old way suffer that it can be incompatible with some other mods. Which is a problem you have got.

If you cannot find solution yourself, jump up to discord, I will try to help you there.