I can't really tell when it happens while in-world (but it definitely occurs when quitting the world), but if you put anything else on a tombstone, it will eventually be erased and the default "Here Lies" will be there. It seems like it happens in-world when I walk far away and return, but it's hard to pinpoint exactly when.

The same happens to me. At first i thought someone else on the server destroyed the Tombstone but after a while i noticed that it happens a couple of times when a i was on the server and near the Tombstone. =/

Edit: Using MCA-1.7.10-4.1.5-universal, RadixCore-1.7.10-1.3.4-universal and Minecraft 1.7.10