I'm  getting straight to the point so, I was playing Minecraft alone I was grinding away then I went to add thorns to my gear, but then all of the past enchants on my gear were gone. This might be somebody trolling me but i'm pretty sure its not i'm a veteran at this game so i know my stuff. Also I could of been dumb because i was using a grind stone while trading with villagers for armor and xp. But it happened twice i had a looting sword i never used a grindstone on my tools but it disappeared in thin air after putting it in my inventory not my hot bar.  Also i dont have real proof since i just bought new diamond gear and repared the isuue so i recreated it on the pictures.