Structure mobs only spawn when using /locate comand but not when finding structure naturally

So I came across this really weird bug that I don't think was here before. Mobs in my structure (setup properly) will not spawn if say I'm walking around and I come across said structure however if I use the /locate command to a further structure said specific mob will load. It's strange.

To reiterate if say I (/place) the structure or did (/locate) the custom mob I had will spawn in said structure in its set position however if I'm exploring or playing the game normally any structure I come across will not have said mob spawned.

Has anyone ever come across this?