I spent two hours last night helping someone out of Atum because they suffocated in the portal on the other side due to some kind of glitch with the automatically generated portal(Valhelsia 2.2.7/Atum 2.1.4), and I had to break the portal for them to get their items out of their grave, only to find out I couldn't relight the portal because water just evaporates. This isn't documented anywhere on the Wiki that I could find. Eventually I just accidentally discovered I was allowed to place water in a ravine and got back out.

Extremely frustrating for a first time experience with the mod to be trapped there with no idea how to get out because there's nothing to indicate that you can place water only below y=50(unless you happen to dig through the config file which I didn't even think to because I was getting attacked too frequently to really do more than a quick google search and figured it was probably like the End and I had to kill a boss to get out)

Maybe portals that generate from coming through from the overworld could be exempt from the water evaporation? Or the portal could be lit with a different liquid or right clicking with the scarab like a nether portal. Or at the very least document the water evaporation mechanic on your mod readme or wiki, but it'd still be incredibly annoying to have to make a new portal in a totally different location just to get out if you have to break the portal for some reason.

A portal can be recreated below Y-level 50 or in an Oasis. Which is working exactly as intended.
The y-level is configurable in the config though, if you wish it to be something different.