So, my friends and I have Schlatt Coins that we use as currency. Each Schlatt Coin is a Gold Nugget, named "Schlatt Coin", which the Lore "The Server's Currency", to ensure that the item is a legitimate Schlatt Coin. We have been having this issue for a while in 1.15.2. Originally, we had a custom villager that can trade Schlatt Coins to and from diamonds. We noticed that the villager often would not work when translating Schlatt Coins back into diamonds. We then started to notice that Schlatt Coins created on different visits to the custom villager would not stack correctly. We then decided that it was not worth it to try and handle the buggy villager, so we got rid of the villager's ability to give items back, as people were losing Schlatt Coins and not getting items back. I then tried to create an item sorting machine at the market which would take items, verify that they were Schlatt Coins, and activate a command block that would give the player diamonds when they dropped Schlatt Coins in the hopper. That didn't work either. It would take Schlatt Coins that were in the original stack that the Schlatt Coins inside the item sorter was from, but none from any other stack.

I will attempt to describe how to replicate the issue, but I will warn you that this is the strangest bug I have ever come across in my 5 years of playing Minecraft. So it may not work as expected originally. Here are the steps you may use to reproduce the issue:

1. Summon a custom villager with the following command: /summon villager ~ ~ ~ {VillagerData: {profession:cleric,level:99,type:plains}

   ,Invulnerable:1,PersistenceRequired:1,Silent:1,NoAI:1,CustomName:"\"Jebidiah Schlatt\"",Offers:{Recipes:[{buy:

   {id:diamond,Count:20}

   ,sell:{id:gold_nugget,Count:1,tag:{display:

   {Name:"\"Schlatt Coin\"",Lore:["\"The Server's Currency\""]}

   }},maxUses:9999999},{buy:{id:gold_nugget,Count:1,tag:{display:

   {Name:"\"Schlatt Coin\"",Lore:["\"The Server's Currency\""]}

   }},sell:{id:diamond,Count:20}

2. Trade 20 diamonds for a Schlatt Coin from the villager
3. Now, use the following command to give yourself a Schlatt Coin: /give @s gold_nugget{display:
   Unknown macro: {Name}

   ]}} 1

4. Attempt to stack the two coins together. It may appear to work, but it may separate when you leave the inventory GUI. It may also work because this bug is incredibly unpredictable.
5. Use the same command in step 3 to generate a third Schlatt Coin, now attempt to trade the Schlatt Coin with the villager. It shouldn't work. You may have to do a little bit of mangling with this one. If it works at first, try and stack two of the Schlatt Coins together, and trade with the villager.

PS. I will note here that we use a Paper server. When experimenting with this issue, I took all of the plugins and datapacks away from the server. I then switched to a vanilla 1.15.2 server. I then verified again with the villager. I also made sure that there were no custom NBT tags from any of the plugins or datapacks which may have interfered with the Schlatt Coins. The only NBT tags we are using are the renaming and the lore.