Greetings!

The reason why the previous version of Dynamis allowed for re-entry was because the Perpetual Hourglass was an actual item that continued to keep track of your Dynamis time even when leaving the area.

As you know, the renewed version of Dynamis requires the key item Prismatic hourglass and because it is not a physical item, it cannot continue to record your Dynamis time should you (or the monsters ) decide you should exit Dynamis.

Time extension key items function the same way as well and are lost upon exit. (Even if you were to get them once again the information from before you exited would be lost.) So with that said, it’s not possible to change this that easily.

While it would definitely be convenient for some of the cases mentioned, it would be better to think of the current Dynamis as one large battlefield, meaning that exiting would end the fight.