WiredTiger clears all the historical versions when a key is updated or removed with no timestamp clear the historical versions.
Fast truncate with no timestamp doesn't remove any of the historical versions. Reinserting the same key again with later timestamps can select the historical versions that should be removed with the truncate command. This issue doesn't occur with cursor->delete.
Fast truncate works on a page level, whereas removing the historical versions are based on the key level, so performing the removal of each historical version using the key defeats the fast truncate.
The possible solution is to raise an error performing the fast truncate on a timestamped table using a no timestamp transaction.