Edit: THIS DOES NOT WORK IF YOU TELEPORT OUT OF THE TREASURE ROOM OR AFTER KILLING THE FINAL BOSS. ONLY DO THIS AFTER COMPLETING A REGULAR ROOM.

I've seen tons of people still confused about this on reddit and Twitch.

Progress is saved after each room so you can portal out of trials and then return. For whatever reason, you can't take the portal straight back but have to use the waypoint and teleport back to the trial. When you enter, it will place you in the same room you were in when you portaled out.

I've done it myself multiple times in Sehkemas.

Edit: This apparently does not work after you complete the trial so don't teleport out of the treasure room!