After much experimentation, I found that this is indeed a collision issue. The interaction trace is being blocked by the collision of my weapon racks. My project has been unmodified for the past 3 months, so something must have changed on the engine/Fortnite side.
I changed my weapon rack collision to per poly, and I can place my weapons as desired.
What is odd is that during live edit, the interaction trace works. It’s just in game they do not.