Playing on latest mod version of 1.20.1 (v5.3.16); I have found a bug related to husbandry and leveling items.
What happened is I was breeding animals and when I would switch to my sword (which had level 3 before this) it gained levels. After a couple minutes of breeding animals the sword got up to level 15. I tested this with other MnS tools and it had the same effect.

That should be normal. Swords are considered husbandry tool. And your sword isn't gaining gear levels, it's gaining tool levels