This has nothing to do with APC.  You are using a web server which launches multiple parent processes.  Each one is going to get its own APC shared memory segment, so when you do an apc_store() you are storing it in the segment belonging to the process you happen to hit and subsequent fetches may or may not hit that same process.

With Apache, there is a single parent process which spawns a bunch of child processes.  The shared memory segment is created in the parent process and inherited by the children so they all see the same one.  I really haven't tested configurations that don't follow this pattern, but you could remove the unlink() call in apc_mmap.c in the "normal filesystem mmap" section there and set apc.mmap_file_mask=/tmp/some_file_name

By not unlinking it, you get a regular file-backed mmap happening without owner-death protection, but multiple processes should hit the same file and thus the same shared memory segment.  In theory anyway.  Let me know if it works, but there is no bug here.  It is more of a feature request to make APC handle non-forking multi-process configurations.