Are axle end caps usually on both ends of the front axle?
You probably lost one. I’m not aware of any design that only uses one end cap, especially when the whole point of having end caps is to convert the hub axle to a useable interface between the hub and fork. Without an end cap, you just have the bare axle exposed. It might also be the wrong length too; some hubs can be converted between boost and non-boost spacing (110 vs 100mm) by switching end caps.