Oh I thought it was broken because I just made a new level with prti and prto, but in which when the stkm enters PRTI he dies at PRTO, without touching anything deadly. That level is still in a sample, though.
Edit:
Check it.
The glitch is also a bit different when in a save instead of in a pasted sample.
Maybe it's a sampling+portal problem? Because I didn't use sample to make the other lvl...