Official comment
In order for the sampler to sync properly to a playing deck, something should be set as the master (either a deck or the sampler itself), and the samples must be loops to be able to sync; one-shot samples CAN NOT SYNC as the intention is so that samples such as drum hits or vocals don't have their play length altered.
The sample (itself) must have the sync enabled and then the sampler must also have the sync enabled in order for everything to match up.
Manually typing in a matched tempo for a non-loop will work, but all you're doing there is changing the beatgrid of the sample, it doesn't affect the playback speed / duration (no stretching / compression).