Apparently some Foundry authors had been creating missions designed to pay out the maximum amount of XP by allowing near-nonstop combat. The patch has slowed that rate down by allowing players to earn only so much XP in five minutes before turning off the XP spigot altogether until the time limit is up.
Players have been reacting to the Foundry nerf with dismay, particularly non-exploit-using authors who feel as though they are being punished with the rest of the class by the XP nerfs (though "legit" farming maps appear to have escaped deletion so far). One player noted called this situation an "endless cat and mouse game" that has happened in other similar systems.
[Thanks to Zaph for the tip!]