PotionExtension is the premise of several mods, but it is backward compatible with this mod. Therefore, we will consider a means to disable this mod while keeping it as a dummy by means of Mixin or other methods.
I think it is possible with a technique like Angelica disabling Optifine
This is a non-priority item. It is an AM2 issue that automatically downloads this one, and I have already created a compatibility improvement fork, so it is not an issue.
However, I think it is a good idea that people who don't know my fork can handle the automatic download by the launcher.

Fixed in the next update, my potion ID extension logic is now a bit safer

Fixed in 1.6.5