When registering a monster in the revscriptsys interface, I will use the barbarian brutetamer as an example, and registering the spell in revscriptsys, gives the following error on the console:

Monster spell tag {name ="barbarian brutetamer skill reducer", interval = 2000, chance = 15, range = 5, target = false}
In the XML interface, the error does not occur.
I tried it and no such error happens.
Actually after testing we noticed that this error came because the monster file was placed inside data/scripts instead of data/monster
so the behaviour is correct and working you just need to place the monster files in the correct folder :D
To elaborate for people stumbling upon this through search;
As far as I know, revscriptsys monster files must be placed inside data/monsters/lua/ folder (if it doesn't exist - create it),
This is different from the normal location of revscriptsys files, which is data/scripts/
data/monster or any subfolder from there on, I've already edited the wiki aswell to have it matching @Znote
Most helpful comment
data/monsteror any subfolder from there on, I've already edited the wiki aswell to have it matching @Znote