Words of arabic origin break the rules I'm no expert but that is what l picked up.
True. It's all described in the tips and notes: https://www.duolingo.com/skill/sw/Adjectives/tips-and-notes.
Why does this not get a prefix in the singular but becomes watajiri in the plural?
Where have you seen watajiri? A lot of the time, even with adjectives that don't need to take prefixes, people will sometimes put a prefix on it, especially to use as a noun, so I can imagine people using watajiri as a noun to mean "rich people" or "the rich".