Issue: In version the listed version (What I was using when I discovered the bug), frontiers that are created will not be saved, and if you exit and come back they will be missing.

Steps to reproduce: Join a world > Create a frontier > Exit > Rejoin > Frontier is missing, including from the menu.

NOTE: Issue is not present in 2.6.0p1, whatever fix was made to stop the crash on startup (Which I haven't experienced) created this issue.

I Initially thought this was a mod incompatibility, but realized it couldn't be, as no other mod (unless you count Journeymap itself), should even interact with this mod.

What mod loader and version are you using?

Quilt 0.26.0.

Fixed in 2.6.0p3.
It was a problem in Common Networking. Thanks to Mysticdrew who solved it very quickly, I wasted most of the time trying to build the project on my PC due to problems with Gradle.

Thanks for your report and sorry for the inconvenience.