As of minecraft update 1.13, mobs on Realms inside any kinds of mob farms seem to spawn normally at first, but after a certain time has passed, the mobs stop spawning completely, forcing you to leave the area to get them spawning again. The areas for those farms are perfectly prepared, removing unwanted spawning spaces outside of the farms themselves. This has affected almost every mob farm as of now (guardian farm, witch farm, pigmen farm and even a drowned farm). Everything worked perfectly during version 1.12.2 and older.