I was in spectator mode, inside of a mineshaft, but the /locate command would only locate one that tried to generate in the ocean, almost 100 blocks away from where I was. I know ocean mineshafts don't generate, but that doesn't explain why /locate couldn't find the one I was in.

Closing because I suspect the issue is that the ocean mineshaft (which doesn't actually generate as you've pointed out) was actually closer than the starting point of the mineshaft you were in. I haven't been able to recreate this issue at all, nor have I heard of others experiencing it.