I think there was an error with variable names in regards to lactation, so that it wasn't turning off or slowing down when it was supposed to, Aric's fixing that in the v09 if i remember correctly. In the mean while, he posted a mini spell/mod someone made to help drain your overfilled girls.
4 years ago(+1)
It is part of the huge farm update (v.9) but that's still squarely in progress. For now, try putting this in Spells.gd and casting it on your girls. That will completely drain them (supposedly, I haven't had my game playable while working on the update to try it yet).
Came here to report the same issue. While we're at it, there should probably be a point where slaves stop producing milk if they aren't pregnant or being milked regularly. I have one slave that had a baby once, and now is constantly producing milk (luckily without this bug, so an enchanted sundress is enough to keep her at low stress, but still). It's also just a lot of busywork to have to milk them through sex or rotating them through the farm, adding an option to allow them to milk themselves (which wastes it but keeps them lactating) would be nice.
There is/was that feature, Swgor. If you go to the expansion.gd file in the mod and go to the option in settings (at the very top) named “lactationstops”, just turn that from false to true (lower case). That will trigger the system that will have a chance to stop lactating daily (increasing in chance every day they aren’t milked).
It’s annoying to have to edit the game files, I know, but after the farm overhaul I’ll work on making settings editable in game