Unknownreported this bug
Currently there is an issue with arrays (Multiple selections) for worlds in Randomspawns. This has impacted a few versions that I know of.
MM Version 5.3.5 | Build# 4356758 | Paper 1.20.1 | Build 62 is the latest I have found.
Error Message: The 'type' attribute must be a valid MythicMob or MythicEntity type.
[20:54:04 WARN]: [MythicMobs] --| Mechanic Line: mobsinradius{t=Mob1,Mob2;r=5;a=<5}
Although it says MythicEntity, this particular error was in fact triggered by adding a new world to the Array. Trying to hunt down more broken builds.