so im not 100% sure this is because of this mod, but i suspect its either the culprit or at least a big contributor

when barreling fluids ie light oil, then unbarrel you get a gain of fluid..
using any quality barrel other then common, you will get a net gain in fluids,
a standard barrel usually takes 50 light oil, but with a legendary barrel it will output 90 light oil.
so setting up a simple loop barrel and unbarrel, you will end up with a net gain

not sure now it might be done by code directly, but i would suggest a method to close the loophole and exploit

barreling required fluids = same as unbarreling fluids
so both scale on quality

Yes, this sounds plausible.

Emptying a barrel is just another recipe as far as the game is concerned. This mod determines whether to award additional fluid based upon the quality of the recipe. So if you use higher quality barrels, emptying them looks like any other higher quality recipe, so you get more output.

Unfortunately, recipes cannot be customized for quality, at least not the last time I looked at the mod API. Otherwise, this mod could be simplified in a lot of ways. It's kind of annoying that barrel quality even exists, but I guess it needs to be there to support barrels as ordinary ingredients in e.g. cliff explosives, which do benefit from quality.

I think the only solution right now is to special-case the barrel-emptying recipes and exclude them from consideration. I will look into that.

Ok, that was easier to fix than I thought. The latest version excludes all empty-xxx-barrel recipes from getting bonus fluids.