I was able to discover why this problem happened and “fix” it.
When you have two Structs (Let’s call them A and B), and Struct A has a variable of Struct B inside of it, and you try to add a new variable in Struct B (the one inside Struct A), after saving Struct B, UE won’t be able to recognize Struct A anymore.
It does know that the variable exists, but it just can’t find it, how to fix it?
Just go back into Struct A (the one that has Struct B that has been changed), and you can see an error where the Struct B variable was, after you find it just remove Struct B and add it again, if done correctly after compiling every other BP that utilizes that Struct A it should all work again just like before.
There is one problem tho, and that is that you will lose every stored default data, and, unfortunately, there is nothing that can be done to recover them, so before you fill your structs with data, make sure that you 100% they have all the variables you want them to have.