Bug report 3,162,085,733: Cannot open an Excavated Chest while Alva is standing on it

Might apply to all Masters, might apply to various other chests, no idea. I'm pretty sure I did Alva's mission before popping the Expedition encounter.

I'm not sure if the game checks whether an Excavated Chest location prevents things from spawning on top of it, but the simplest explanation would be the game didn't check that (or didn't check it correctly) and thus it allowed them to be on the same tile.