The arabian factions got new building chains with the Desert Building DLC, but for Arabian Levy Spearmen, Arabian Slingers and Arabian Levy Skirmishers the entries in the building_units_allowed_table
for almost all buildings, which allowed their former recruitment, are missing.
So Gerrha, Ma'in, Mascat and Qidri can´t recruit this basic units.
I have created a small bug fix mod for this:https://steamcommunity.com/sharedfiles/filedetails/?id=1354256001&tscn=1532235883
Hopefully you can fix this with the ancestral patch.