https://github.com/AHilyard/MerchantMarkers/blob/1.16.5/src/main/java/com/anthonyhilyard/merchantmarkers/render/Markers.java

At line 150: if (entity instanceof AbstractVillagerEntity)

If you detect IMerchant instead of AbstractVillagerEntity, this mod can support merchants from more mods. For example, the Goblin Merchant, whose goblin entity does not inherit AbstractVillagerEntity because it does not have Age, but goblin implements IMerchant.

In fact, after I modified this code with mixin, the whole mod worked quite well...

oh sorry it's my fault...

Well I would suggest that to the author of that mod. Using a Mixin to patch another mod is not a good practice. I am all for adding support, don't get me wrong.

Edit: I believe you posted on the wrong repo based on the wording.