Millstone can place multiple items. But, millstone only processes one item when place multiple items. In this case, I think the millstone needs to be processed continuously.
This is the CraftTweaker script used in the video.
<recipetype:primutils:milling>.addJSONRecipe("pj_milling_plant2flour", { "input": { "tag": "forge:flour_plants" }, "total_progress": 2, "output": [ { "result": { "item": "pamhc2foodcore:flouritem" }, "chance": 100 } ] });
(I'm not English speaker, so it may be an unnatural sentence.)

Thanks for reporting, I'll take a look. :)

Ok, I think this should fix it, can you try it out before I publish the changes? https://github.com/W1nter-isHere/PrimitiveUtilities/releases/tag/Beta

It works perfectly!
Thank you for your support!

No problem, glad it works. The changes will be published.