You mentioned you were using the Android port.

Did you recently update it? It hasn't been updated in a few months, but I heard a report once about an update wiping stamps.

Can you connect your phone to a computer, or use a file manager on the phone, and go to /root/sdcard/Android/data/uk.co.powdertoy.tpt/files/
Inside there, there should be a folder "stamps" with all your stamps. How many are in there? If this folder was somehow deleted, then your stamps would also be deleted. This could happen if you recently uninstalled tpt.

If "rescan stamps" didn't work though, it's possible the stamps were deleted.