Linux – eCryptfs with dropbox: must remount for synchronized changes to be visible

I am experimenting with using eCryptfs on top of dropbox, and I am encountering some issues.

My system is GNU/Linux, openSUSE 12.2 to be exact.

My setup is thus:
I have set up two instances of VirtualBox running openSUSE 12.2, lets call them VM1 and VM2. Dropbox, as usual, is synchronizing everything in ~/Dropbox.
To create my eCryptfs setup I do the following on both the VMs:

mkdir -m 500 ~/ecryptfs_upper
mkdir -m 700 ~/Dropbox/ecryptfs_lower
sudo mount -t ecryptfs Dropbox/ecryptfs_lower/ ecryptfs_upper/

I configure eCryptfs with:

key type: passphrase
cipher: aes
key bytes: 16
plaintext passthrough: no
filename encryption: yes

If I now proceed to create a file in ~/ecryptfs_upper on VM1, it will show up correctly on VM2 as well. However when I then change this file on one VM it sometimes (often but not always for some reason) will not appear to be updated on the other VM.

If I inspect the underlying files in ~/Dropbox/ecryptfs_lower on the two VMs they are identical (sha256sum produces the same hash), so dropbox has correctly managed to synchronize them. But the corresponding files in ~/ecryptfs_upper are still different! I have to umount then again mount eCryptfs to have the changes show up correctly.

The problem seems to be that when dropbox updates a file in the eCryptfs lower directory, eCryptfs doesn't notice the change. Presumably eCryptfs is assuming that all changes will go through the mount. For most use cases that is obviously a fair assumption, but when using eCryptfs to encrypt synchronized cloud storage like dropbox, it is obviously a big problem.

I've seen several people advocating using eCryptfs with dropbox, but I haven't seen this problem mentioned. Does anyone know of a fix (a way to turn off the cache that eCryptfs seems to be using for example), or of some alternative to eCryptfs that would not suffer from this problem?