When using the mission editor, attempting to save the mission gives a warning about publishing to production; the save mission as copy works as intended.

Is this a mission you're working on, that isn't in production?

Regular PCC members have never been able to save over missions in production.

This is a personal mission that i'm working on; it is not nor should it be in production. I tried making a new mission and had the same issue when trying to save that one via the normal save button; I also asked on the PCC section of the discord, and "Allepac Inati" claims to have the same issue.

The warning was accidentally being shown for all missions. Accepting the warning properly saves the mission to the test server.
The bug has been fixed. Do a force-refresh of the page to get the fix.