@ate0ate: I found the issue, fixed it (hopefully) and uploaded the new version to github. I'm betting this bug was there in a19 as well...
Your idea felt right (I thought I had gotten something cut in half) until I found what it was. The bug was weird but it took a bit for me to find as I'm not too familiar with how buffs work. I had a variable in the wrong place and it was always using my maximum infection counter time which is 3 hours for one calculation, and the RNG roll for infection you get for everything else. The "cure" rates weren't being cut in half, it just appeared that way due to a few ways math and RNG hit more often. Once I tested more I was seeing some cure rates were being cut into fourths and other odd small numbers....
Having looked at that... I also just noticed this in case anyone runs into this: If you take a "cure" (like honey) and the infection % and cure % look identical in the UI: the cure might not completely cure the infection. Its a close race but it looks like it can happen ( probably a tiny bit less rare with my mod loaded). It has to do how the game does infection and is not a bug. The "infection" and "cure" math doesn't work on "percent" with 1 decimal place like it shows in game, it has more decimal places in reality. This means that you may see "you have 5.0% percent infection" but in reality it might be 5.09% so when you take the honey cure (5%, well it has all zeros: 5.000%) you can "see" it giving 5%, but when you hit 0.0% in the game UI it might actually be 0.09% (or something like that) so the honey won't cure it completely and it can climb back up. Just pointing this out as its a likely rare edge case and the game normally works like this BUT since my mod adds to the "infection speed" a lot so its possible you might run into this edge case more often than in vanilla Even this description is a bit off on whats actually happening but its close enough.
Also: I might be wrong about this, just what I think can happen given the digits shown in the UI are cut off from the behind the scenes math which has more digits. Maybe the game compensates for this with rounding or something and I'm not seeing it.